# System Catalogs

## Overview

The system catalogs are the place where a relational database management system stores schema metadata, such as information about tables and columns, and internal bookkeeping information. PostgreSQL's system catalogs are regular tables. You can drop and recreate the tables, add columns, insert and update values, and severely mess up your system that way. Normally one should not change the system catalogs by hand, there are always SQL commands to do that. (For example, `CREATE DATABASE` inserts a row into the pg_database catalog -- and actually creates the database on disk.) There are some exceptions for especially esoteric operations, such as adding index access methods.

Most system catalogs are copied from the template database during database creation, and are thereafter database-specific. A few catalogs are physically shared across all databases in an installation; these are marked in the descriptions of the individual catalogs.

## System Catalogs

Catalog Name | Purpose
pg_aggregate | aggregate functions
pg_am | index access methods
pg_amop | access method operators
pg_amproc | access method support procedures
pg_attrdef | column default values
pg_attribute | table columns (attributes, fields)
pg_cast | casts (data type conversions)
pg_class | tables, indexes, sequences (relations)
pg_constraint | check constraints, unique / primary key constraints, foreign key constraints
pg_conversion | encoding conversion information
pg_database | databases within this database cluster
pg_depend | dependencies between database objects
pg_description | descriptions or comments on database objects
pg_group | groups of database users
pg_index | additional index information
pg_inherits | table inheritance hierarchy
pg_language | languages for writing functions
pg_largeobject | large objects
pg_listener | asynchronous notification
pg_namespace | namespaces (schemas)
pg_opclass | index access method operator classes
pg_operator | operators
pg_proc | functions and procedures
pg_rewrite | query rewriter rules
pg_shadow | database users
pg_statistic | optimizer statistics
pg_trigger | triggers
pg_type | data types

More detailed documentation of each catalog follows below.

## pg_aggregate

pg_aggregate stores information about aggregate functions. An aggregate function is a function that operates on a set of values (typically one column from each row that matches a query condition) and returns a single value computed from all these values. Typical aggregate functions are `sum`, `count`, and `max`. Each entry in pg_aggregate is an extension of an entry in pg_proc. The pg_proc entry carries the aggregate's name, input and output datatypes, and other information that is similar to ordinary functions.

## pg_aggregate Columns

Name | Type | References | Description
aggfnoid | `regproc` | pg_proc.oid | pg_proc OID of the aggregate function
aggtransfn | `regproc` | pg_proc.oid | Transition function
aggfinalfn | `regproc` | pg_proc.oid | Final function (zero if none)
aggtranstype | `oid` | pg_type.oid | The type of the aggregate function's internal transition (state) data
agginitval | `text` |  | The initial value of the transition state. This is a text field containing the initial value in its external string representation. If the field is NULL, the transition state value starts out NULL.

New aggregate functions are registered with the `CREATE AGGREGATE` command. See the Programmer's Guide for more information about writing aggregate functions and the meaning of the transition functions, etc.

## pg_am

pg_am stores information about index access methods. There is one row for each index access method supported by the system.

## pg_am Columns

Name | Type | References | Description
amname | `name` |  | name of the access method
amowner | `int4` | pg_shadow.usesysid | user ID of the owner (currently not used)
amstrategies | `int2` |  | number of operator strategies for this access method
amsupport | `int2` |  | number of support routines for this access method
amorderstrategy | `int2` |  | zero if the index offers no sort order, otherwise the strategy number of the strategy operator that describes the sort order
amcanunique | `bool` |  | does AM support unique indexes?
amcanmulticol | `bool` |  | does AM support multicolumn indexes?
amindexnulls | `bool` |  | does AM support NULL index entries?
amconcurrent | `bool` |  | does AM support concurrent updates?
amgettuple | `regproc` | pg_proc.oid | next valid tuple function
aminsert | `regproc` | pg_proc.oid | insert this tuple function
ambeginscan | `regproc` | pg_proc.oid | start new scan function
amrescan | `regproc` | pg_proc.oid | restart this scan function
amendscan | `regproc` | pg_proc.oid | end this scan function
ammarkpos | `regproc` | pg_proc.oid | mark current scan position function
amrestrpos | `regproc` | pg_proc.oid | restore marked scan position function
ambuild | `regproc` | pg_proc.oid | build new index function
ambulkdelete | `regproc` | pg_proc.oid | bulk-delete function
amcostestimate | `regproc` | pg_proc.oid | estimate cost of an indexscan

An index AM that supports multiple columns (has amcanmulticol true) must support indexing nulls in columns after the first, because the planner will assume the index can be used for queries on just the first column(s). For example, consider an index on (a,b) and a query WHERE a = 4. The system will assume the index can be used to scan for rows with a = 4, which is wrong if the index omits rows where b is null. However it is okay to omit rows where the first indexed column is null. (GiST currently does so.) amindexnulls should be set true only if the index AM indexes all rows, including arbitrary combinations of nulls.

## pg_amop

pg_amop stores information about operators associated with index access method operator classes. There is one row for each operator that is a member of an operator class.

## pg_amop Columns

Name | Type | References | Description
amopclaid | `oid` | pg_opclass.oid | the index opclass this entry is for
amopstrategy | `int2` |  | operator strategy number
amopreqcheck | `bool` |  | index hit must be rechecked
amopopr | `oid` | pg_operator.oid | the operator's pg_operator OID

## pg_amproc

pg_amproc stores information about support procedures associated with index access method operator classes. There is one row for each support procedure belonging to an operator class.

## pg_amproc Columns

Name | Type | References | Description
amopclaid | `oid` | pg_opclass.oid | the index opclass this entry is for
amprocnum | `int2` |  | support procedure index
amproc | `regproc` | pg_proc.oid | OID of the proc

## pg_attrdef

This catalog stores column default values. The main information about columns is stored in pg_attribute (see below). Only columns that explicitly specify a default value (when the table is created or the column is added) will have an entry here.

## pg_attrdef Columns

Name | Type | References | Description
adrelid | `oid` | pg_class.oid | The table this column belongs to
adnum | `int2` | pg_attribute.attnum | The number of the column
adbin | `text` |  | An internal representation of the column default value
adsrc | `text` |  | A human-readable representation of the default value

## pg_attribute

pg_attribute stores information about table columns. There will be exactly one pg_attribute row for every column in every table in the database. (There will also be attribute entries for indexes and other objects. See pg_class.)

The term attribute is equivalent to column and is used for historical reasons.

## pg_attribute Columns

Name | Type | References | Description
attrelid | `oid` | pg_class.oid | The table this column belongs to
attname | `name` |  | Column name
atttypid | `oid` | pg_type.oid | The data type of this column
attstattarget | `int4` |  | attstattarget controls the level of detail of statistics accumulated for this column by `ANALYZE`. A zero value indicates that no statistics should be collected. A negative value says to use the system default statistics target. The exact meaning of positive values is datatype-dependent. For scalar datatypes, attstattarget is both the target number of most common values to collect, and the target number of histogram bins to create.
attlen | `int2` |  | This is a copy of pg_type.typlen of this column's type.
attnum | `int2` |  | The number of the column. Ordinary columns are numbered from 1 up. System columns, such as oid, have (arbitrary) negative numbers.
attndims | `int4` |  | Number of dimensions, if the column is an array type; otherwise 0. (Presently, the number of dimensions of an array is not enforced, so any nonzero value effectively means it's an array.)
attcacheoff | `int4` |  | Always -1 in storage, but when loaded into a tuple descriptor in memory this may be updated to cache the offset of the attribute within the tuple.
atttypmod | `int4` |  | atttypmod records type-specific data supplied at table creation time (for example, the maximum length of a `varchar` column). It is passed to type-specific input functions and length coercion functions. The value will generally be -1 for types that do not need typmod.
attbyval | `bool` |  | A copy of pg_type.typbyval of this column's type
attstorage | `char` |  | Normally a copy of pg_type.typstorage of this column's type. For TOASTable datatypes, this can be altered after column creation to control storage policy.
attisset | `bool` |  | If true, this attribute is a set. In that case, what is really stored in the attribute is the OID of a tuple in the pg_proc catalog. The pg_proc tuple contains the query string that defines this set - i.e., the query to run to get the set. So the atttypid (see above) refers to the type returned by this query, but the actual length of this attribute is the length (size) of an `oid`. --- At least this is the theory. All this is probably quite broken these days.
attalign | `char` |  | A copy of pg_type.typalign of this column's type
attnotnull | `bool` |  | This represents a NOT NULL constraint. It is possible to change this field to enable or disable the constraint.
atthasdef | `bool` |  | This column has a default value, in which case there will be a corresponding entry in the pg_attrdef catalog that actually defines the value.
attisdropped | `bool` |  | This column has been dropped and is no longer valid. A dropped column is still physically present in the table, but is ignored by the parser and so cannot be accessed via SQL.
attislocal | `bool` |  | This column is defined locally in the relation. Note that a column may be locally defined and inherited simultaneously.
attinhcount | `int4` |  | The number of direct ancestors this column has. A column with a nonzero number of ancestors cannot be dropped nor renamed.

## pg_cast

pg_cast stores data type conversion paths, both built-in paths and those defined with `CREATE CAST`.

## pg_cast Columns

Name | Type | References | Description
castsource | `oid` | pg_type.oid | OID of the source data type
casttarget | `oid` | pg_type.oid | OID of the target data type
castfunc | `oid` | pg_proc.oid | The OID of the function to use to perform this cast. Zero is stored if the data types are binary coercible (that is, no run-time operation is needed to perform the cast).
castcontext | `char` |  | Indicates what contexts the cast may be invoked in. `e` means only as an explicit cast (using `CAST`, `::`, or function-call syntax). `a` means implicitly in assignment to a target column, as well as explicitly. `i` means implicitly in expressions, as well as the other cases.

## pg_class

pg_class catalogs tables and most everything else that has columns or is otherwise similar to a table. This includes indexes (but see also pg_index), sequences, views, and some kinds of special relation; see relkind. Below, when we mean all of these kinds of objects we speak of relations. Not all fields are meaningful for all relation types.

## pg_class Columns

Name | Type | References | Description
relname | `name` |  | Name of the table, index, view, etc.
relnamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this relation
reltype | `oid` | pg_type.oid | The OID of the data type that corresponds to this table, if any (zero for indexes, which have no pg_type entry)
relowner | `int4` | pg_shadow.usesysid | Owner of the relation
relam | `oid` | pg_am.oid | If this is an index, the access method used (B-tree, hash, etc.)
relfilenode | `oid` |  | Name of the on-disk file of this relation; 0 if none
relpages | `int4` |  | Size of the on-disk representation of this table in pages (size BLCKSZ). This is only an estimate used by the planner. It is updated by `VACUUM`, `ANALYZE`, and `CREATE INDEX`.
reltuples | `float4` |  | Number of tuples in the table. This is only an estimate used by the planner. It is updated by `VACUUM`, `ANALYZE`, and `CREATE INDEX`.
reltoastrelid | `oid` | pg_class.oid | OID of the TOAST table associated with this table, 0 if none. The TOAST table stores large attributes out of line in a secondary table.
reltoastidxid | `oid` | pg_class.oid | For a TOAST table, the OID of its index. 0 if not a TOAST table.
relhasindex | `bool` |  | True if this is a table and it has (or recently had) any indexes. This is set by CREATE INDEX, but not cleared immediately by DROP INDEX. VACUUM clears relhasindex if it finds the table has no indexes.
relisshared | `bool` |  | True if this table is shared across all databases in the cluster. Only certain system catalogs (such as pg_database) are shared.
relkind | `char` |  | 'r' = ordinary table, 'i' = index, 'S' = sequence, 'v' = view, 'c' = composite type, 's' = special, 't' = TOAST table
relnatts | `int2` |  | Number of user columns in the relation (system columns not counted). There must be this many corresponding entries in pg_attribute. See also pg_attribute.attnum.
relchecks | `int2` |  | Number of check constraints on the table; see pg_constraint catalog
reltriggers | `int2` |  | Number of triggers on the table; see pg_trigger catalog
relukeys | `int2` |  | unused (Not the number of unique keys)
relfkeys | `int2` |  | unused (Not the number of foreign keys on the table)
relrefs | `int2` |  | unused
relhasoids | `bool` |  | True if we generate an OID for each row of the relation.
relhaspkey | `bool` |  | True if the table has (or once had) a primary key.
relhasrules | `bool` |  | Table has rules; see pg_rewrite catalog
relhassubclass | `bool` |  | At least one table inherits from this one
relacl | `aclitem[]` |  | Access permissions. See the descriptions of `GRANT` and `REVOKE` for details.

## pg_constraint

This system catalog stores CHECK, PRIMARY KEY, UNIQUE, and FOREIGN KEY constraints on tables. (Column constraints are not treated specially. Every column constraint is equivalent to some table constraint.) See under `CREATE TABLE` for more information.

NOT NULL constraints are represented in the pg_attribute catalog.

CHECK constraints on domains are stored here, too. Global ASSERTIONS (a currently-unsupported SQL feature) may someday appear here as well.

## pg_constraint Columns

Name | Type | References | Description
conname | `name` |  | Constraint name (not necessarily unique!)
connamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this constraint
contype | `char` |  | 'c' = check constraint, 'f' = foreign key constraint, 'p' = primary key constraint, 'u' = unique constraint
condeferrable | `boolean` |  | Is the constraint deferrable?
condeferred | `boolean` |  | Is the constraint deferred by default?
conrelid | `oid` | pg_class.oid | The table this constraint is on; 0 if not a table constraint
contypid | `oid` | pg_type.oid | The domain this constraint is on; 0 if not a domain constraint
confrelid | `oid` | pg_class.oid | If a foreign key, the referenced table; else 0
confupdtype | `char` |  | Foreign key update action code
confdeltype | `char` |  | Foreign key deletion action code
confmatchtype | `char` |  | Foreign key match type
conkey | `int2[]` | pg_attribute.attnum | If a table constraint, list of columns which the constraint constrains
confkey | `int2[]` | pg_attribute.attnum | If a foreign key, list of the referenced columns
conbin | `text` |  | If a check constraint, an internal representation of the expression
consrc | `text` |  | If a check constraint, a human-readable representation of the expression

pg_class.relchecks needs to agree with the number of check-constraint entries found in this table for the given relation.

## pg_conversion

This system catalog stores encoding conversion information. See `CREATE CONVERSION` for more information.

## pg_conversion Columns

Name | Type | References | Description
conname | `name` |  | Conversion name (unique within a namespace)
connamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this conversion
conowner | `int4` | pg_shadow.usesysid | Owner (creator) of the namespace
conforencoding | `int4` |  | Source(for) encoding ID
contoencoding | `int4` |  | Destination(to) encoding ID
conproc | `regproc` | pg_proc.oid | Conversion procedure
condefault | `boolean` |  | true if this is the default conversion

## pg_database

The pg_database catalog stores information about the available databases. Databases are created with the `CREATE DATABASE` command. Consult the Administrator's Guide for details about the meaning of some of the parameters.

Unlike most system catalogs, pg_database is shared across all databases of a cluster: there is only one copy of pg_database per cluster, not one per database.

## pg_database Columns

Name | Type | References | Description
datname | `name` |  | Database name
datdba | `int4` | pg_shadow.usesysid | Owner of the database, usually the user who created it
encoding | `int4` |  | Character/multibyte encoding for this database
datistemplate | `bool` |  | If true then this database can be used in the TEMPLATE clause of `CREATE DATABASE` to create a new database as a clone of this one.
datallowconn | `bool` |  | If false then no one can connect to this database. This is used to protect the template0 database from being altered.
datlastsysoid | `oid` |  | Last system OID in the database; useful particularly to pg_dump
datvacuumxid | `xid` |  | All tuples inserted or deleted by transaction IDs before this one have been marked as known committed or known aborted in this database. This is used to determine when commit-log space can be recycled.
datfrozenxid | `xid` |  | All tuples inserted by transaction IDs before this one have been relabeled with a permanent (frozen) transaction ID in this database. This is useful to check whether a database must be vacuumed soon to avoid transaction ID wraparound problems.
datpath | `text` |  | If the database is stored at an alternative location then this records the location. It's either an environment variable name or an absolute path, depending how it was entered.
datconfig | `text[]` |  | Session defaults for run-time configuration variables
datacl | `aclitem[]` |  | Access permissions

## pg_depend

The pg_depend table records the dependency relationships between database objects. This information allows `DROP` commands to find which other objects must be dropped by `DROP CASCADE`, or prevent dropping in the `DROP RESTRICT` case.

## pg_depend Columns

Name | Type | References | Description
classid | `oid` | pg_class.oid | The oid of the system catalog the dependent object is in
objid | `oid` | any oid attribute | The oid of the specific dependent object
objsubid | `int4` |  | For a table attribute, this is the attribute's column number (the objid and classid refer to the table itself). For all other object types, this field is presently zero.
refclassid | `oid` | pg_class.oid | The oid of the system catalog the referenced object is in
refobjid | `oid` | any oid attribute | The oid of the specific referenced object
refobjsubid | `int4` |  | For a table attribute, this is the attribute's column number (the refobjid and refclassid refer to the table itself). For all other object types, this field is presently zero.
deptype | `char` |  | A code defining the specific semantics of this dependency relationship.

In all cases, a pg_depend entry indicates that the referenced object may not be dropped without also dropping the dependent object. However, there are several subflavors identified by deptype:

- DEPENDENCY_NORMAL ('n'): normal relationship between separately-created objects. The dependent object may be dropped without affecting the referenced object. The referenced object may only be dropped by specifying CASCADE, in which case the dependent object is dropped too. Example: a table column has a normal dependency on its datatype.

- DEPENDENCY_AUTO ('a'): the dependent object can be dropped separately from the referenced object, and should be automatically dropped (regardless of RESTRICT or CASCADE mode) if the referenced object is dropped. Example: a named constraint on a table is made auto-dependent on the table, so that it will go away if the table is dropped.

- DEPENDENCY_INTERNAL ('i'): the dependent object was created as part of creation of the referenced object, and is really just a part of its internal implementation. A DROP of the dependent object will be disallowed outright (we'll tell the user to issue a DROP against the referenced object, instead). A DROP of the referenced object will be propagated through to drop the dependent object whether CASCADE is specified or not. Example: a trigger that's created to enforce a foreign-key constraint is made internally dependent on the constraint's pg_constraint entry.

- DEPENDENCY_PIN ('p'): there is no dependent object; this type of entry is a signal that the system itself depends on the referenced object, and so that object must never be deleted. Entries of this type are created only during initdb. The fields for the dependent object contain zeroes.

Other dependency flavors may be needed in future.

## pg_description

The pg_description table can store an optional description or comment for each database object. Descriptions can be manipulated with the `COMMENT` command and viewed with psql's `\d` commands. Descriptions of many built-in system objects are provided in the initial contents of pg_description.

## pg_description Columns

Name | Type | References | Description
objoid | `oid` | any oid attribute | The oid of the object this description pertains to
classoid | `oid` | pg_class.oid | The oid of the system catalog this object appears in
objsubid | `int4` |  | For a comment on a table attribute, this is the attribute's column number (the objoid and classoid refer to the table itself). For all other object types, this field is presently zero.
description | `text` |  | Arbitrary text that serves as the description of this object.

## pg_group

This catalog defines groups and stores what users belong to what groups. Groups are created with the `CREATE GROUP` command. Consult the Administrator's Guide for information about user permission management.

Because user and group identities are cluster-wide, pg_group is shared across all databases of a cluster: there is only one copy of pg_group per cluster, not one per database.

## pg_group Columns

Name | Type | References | Description
groname | `name` |  | Name of the group
grosysid | `int4` |  | An arbitrary number to identify this group
grolist | `int4[]` | pg_shadow.usesysid | An array containing the ids of the users in this group

## pg_index

pg_index contains part of the information about indexes. The rest is mostly in pg_class.

## pg_index Columns

Name | Type | References | Description
indexrelid | `oid` | pg_class.oid | The OID of the pg_class entry for this index
indrelid | `oid` | pg_class.oid | The OID of the pg_class entry for the table this index is for
indproc | `regproc` | pg_proc.oid | The function's OID if this is a functional index, else zero
indkey | `int2vector` | pg_attribute.attnum | This is a vector (array) of up to INDEX_MAX_KEYS values that indicate which table columns this index pertains to. For example a value of `1 3` would mean that the first and the third column make up the index key. For a functional index, these columns are the inputs to the function, and the function's return value is the index key.
indclass | `oidvector` | pg_opclass.oid | For each column in the index key this contains a reference to the operator class to use. See pg_opclass for details.
indisclustered | `bool` |  | If true, the table was last clustered on this index.
indisunique | `bool` |  | If true, this is a unique index.
indisprimary | `bool` |  | If true, this index represents the primary key of the table. (indisunique should always be true when this is true.)
indreference | `oid` |  | unused
indpred | `text` |  | Expression tree (in the form of a nodeToString representation) for partial index predicate. Empty string if not a partial index.

## pg_inherits

This catalog records information about table inheritance hierarchies.

## pg_inherits Columns

Name | Type | References | Description
inhrelid | `oid` | pg_class.oid | The OID of the child table.
inhparent | `oid` | pg_class.oid | The OID of the parent table.
inhseqno | `int4` |  | If there is more than one parent for a child table (multiple inheritance), this number tells the order in which the inherited columns are to be arranged. The count starts at 1.

## pg_language

pg_language registers call interfaces or languages in which you can write functions or stored procedures. See under `CREATE LANGUAGE` and in the Programmer's Guide for more information about language handlers.

## pg_language Columns

Name | Type | References | Description
lanname | `name` |  | Name of the language (to be specified when creating a function)
lanispl | `bool` |  | This is false for internal languages (such as SQL) and true for user-defined languages. Currently, pg_dump still uses this to determine which languages need to be dumped, but this may be replaced by a different mechanism sometime.
lanpltrusted | `bool` |  | This is a trusted language. See under `CREATE LANGUAGE` what this means. If this is an internal language (lanispl is false) then this field is meaningless.
lanplcallfoid | `oid` | pg_proc.oid | For non-internal languages this references the language handler, which is a special function that is responsible for executing all functions that are written in the particular language.
lanvalidator | `oid` | pg_proc.oid | This references a language validator function that is responsible for checking the syntax and validity of new functions when they are created. See under `CREATE LANGUAGE` for further information about validators.
lanacl | `aclitem[]` |  | Access permissions

## pg_largeobject

pg_largeobject holds the data making up large objects. A large object is identified by an OID assigned when it is created. Each large object is broken into segments or pages small enough to be conveniently stored as rows in pg_largeobject. The amount of data per page is defined to be LOBLKSIZE (which is currently BLCKSZ/4, or typically 2Kbytes).

## pg_largeobject Columns

Name | Type | References | Description
loid | `oid` |  | Identifier of the large object that includes this page
pageno | `int4` |  | Page number of this page within its large object (counting from zero)
data | `bytea` |  | Actual data stored in the large object. This will never be more than LOBLKSIZE bytes, and may be less.

Each row of pg_largeobject holds data for one page of a large object, beginning at byte offset (pageno * LOBLKSIZE) within the object. The implementation allows sparse storage: pages may be missing, and may be shorter than LOBLKSIZE bytes even if they are not the last page of the object. Missing regions within a large object read as zeroes.

## pg_listener

pg_listener supports the `LISTEN` and `NOTIFY` commands. A listener creates an entry in pg_listener for each notification name it is listening for. A notifier scans pg_listener and updates each matching entry to show that a notification has occurred. The notifier also sends a signal (using the PID recorded in the table) to awaken the listener from sleep.

## pg_listener Columns

Name | Type | References | Description
relname | `name` |  | Notify condition name. (The name need not match any actual relation in the database; the term relname is historical.)
listenerpid | `int4` |  | PID of the backend process that created this entry.
notification | `int4` |  | Zero if no event is pending for this listener. If an event is pending, the PID of the backend that sent the notification.

## pg_namespace

A namespace is the structure underlying SQL92 schemas: each namespace can have a separate collection of relations, types, etc without name conflicts.

## pg_namespace Columns

Name | Type | References | Description
nspname | `name` |  | Name of the namespace
nspowner | `int4` | pg_shadow.usesysid | Owner (creator) of the namespace
nspacl | `aclitem[]` |  | Access permissions

## pg_opclass

pg_opclass defines index access method operator classes. Each operator class defines semantics for index columns of a particular datatype and a particular index access method. Note that there can be multiple operator classes for a given datatype/access method combination, thus supporting multiple behaviors.

Operator classes are described at length in the Programmer's Guide.

## pg_opclass Columns

Name | Type | References | Description
opcamid | `oid` | pg_am.oid | index access method opclass is for
opcname | `name` |  | name of this opclass
opcnamespace | `oid` | pg_namespace.oid | namespace of this opclass
opcowner | `int4` | pg_shadow.usesysid | opclass owner
opcintype | `oid` | pg_type.oid | type of input data for opclass
opcdefault | `bool` |  | true if opclass is default for opcintype
opckeytype | `oid` | pg_type.oid | type of index data, or zero if same as opcintype

The majority of the information defining an operator class is actually not in its pg_opclass row, but in the associated rows in pg_amop and pg_amproc. Those rows are considered to be part of the operator class definition --- this is not unlike the way that a relation is defined by a single pg_class row, plus associated rows in pg_attribute and other tables.

## pg_operator

See `CREATE OPERATOR` and the Programmer's Guide for details on these operator parameters.

## pg_operator Columns

Name | Type | References | Description
oprname | `name` |  | Name of the operator
oprnamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this operator
oprowner | `int4` | pg_shadow.usesysid | Owner (creator) of the operator
oprkind | `char` |  | 'b' = infix (both), 'l' = prefix (left), 'r' = postfix (right)
oprcanhash | `bool` |  | This operator supports hash joins.
oprleft | `oid` | pg_type.oid | Type of the left operand
oprright | `oid` | pg_type.oid | Type of the right operand
oprresult | `oid` | pg_type.oid | Type of the result
oprcom | `oid` | pg_operator.oid | Commutator of this operator, if any
oprnegate | `oid` | pg_operator.oid | Negator of this operator, if any
oprlsortop | `oid` | pg_operator.oid | If this operator supports merge joins, the operator that sorts the type of the left-hand operand (`L<L`)
oprrsortop | `oid` | pg_operator.oid | If this operator supports merge joins, the operator that sorts the type of the right-hand operand (`R<R`)
oprltcmpop | `oid` | pg_operator.oid | If this operator supports merge joins, the less-than operator that compares the left and right operand types (`L<R`)
oprgtcmpop | `oid` | pg_operator.oid | If this operator supports merge joins, the greater-than operator that compares the left and right operand types (`L>R`)
oprcode | `regproc` | pg_proc.oid | Function that implements this operator
oprrest | `regproc` | pg_proc.oid | Restriction selectivity estimation function for this operator
oprjoin | `regproc` | pg_proc.oid | Join selectivity estimation function for this operator

Unused fields contain zeroes, for example oprleft is zero for a prefix operator.

## pg_proc

This catalog stores information about functions (or procedures). The description of `CREATE FUNCTION` and the Programmer's Guide contain more information about the meaning of some fields.

The table contains data for aggregate functions as well as plain functions. If proisagg is true, there should be a matching row in pg_aggregate.

## pg_proc Columns

Name | Type | References | Description
proname | `name` |  | Name of the function
pronamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this function
proowner | `int4` | pg_shadow.usesysid | Owner (creator) of the function
prolang | `oid` | pg_language.oid | Implementation language or call interface of this function
proisagg | `bool` |  | Function is an aggregate function
prosecdef | `bool` |  | Function is a security definer (i.e., a setuid function)
proisstrict | `bool` |  | Function returns null if any call argument is null. In that case the function won't actually be called at all. Functions that are not strict must be prepared to handle null inputs.
proretset | `bool` |  | Function returns a set (ie, multiple values of the specified data type)
provolatile | `char` |  | provolatile tells whether the function's result depends only on its input arguments, or is affected by outside factors. It is `i` for immutable functions, which always deliver the same result for the same inputs. It is `s` for stable functions, whose results (for fixed inputs) do not change within a scan. It is `v` for volatile functions, whose results may change at any time. (Use `v` also for functions with side-effects, so that calls to them cannot get optimized away.)
pronargs | `int2` |  | Number of arguments
prorettype | `oid` | pg_type.oid | Data type of the return value
proargtypes | `oidvector` | pg_type.oid | A vector with the data types of the function arguments
prosrc | `text` |  | This tells the function handler how to invoke the function. It might be the actual source code of the function for interpreted languages, a link symbol, a file name, or just about anything else, depending on the implementation language/call convention.
probin | `bytea` |  | Additional information about how to invoke the function. Again, the interpretation is language-specific.
proacl | `aclitem[]` |  | Access permissions

Currently, prosrc contains the function's C-language name (link symbol) for compiled functions, both built-in and dynamically loaded. For all other language types, prosrc contains the function's source text.

Currently, probin is unused except for dynamically-loaded C functions, for which it gives the name of the shared library file containing the function.

## pg_rewrite

This system catalog stores rewrite rules for tables and views.

## pg_rewrite Columns

Name | Type | References | Description
rulename | `name` |  | Rule name
ev_class | `oid` | pg_class.oid | The table this rule is for
ev_attr | `int2` |  | The column this rule is for (currently, always zero to indicate the whole table)
ev_type | `char` |  | Event type that the rule is for: '1' = SELECT, '2' = UPDATE, '3' = INSERT, '4' = DELETE
is_instead | `bool` |  | True if the rule is an INSTEAD rule
ev_qual | `text` |  | Expression tree (in the form of a nodeToString representation) for the rule's qualifying condition
ev_action | `text` |  | Query tree (in the form of a nodeToString representation) for the rule's action

pg_class.relhasrules must be true if a table has any rules in this catalog.

## pg_shadow

pg_shadow contains information about database users. The name stems from the fact that this table should not be readable by the public since it contains passwords. pg_user is a publicly readable view on pg_shadow that blanks out the password field.

The Administrator's Guide contains detailed information about user and permission management.

Because user identities are cluster-wide, pg_shadow is shared across all databases of a cluster: there is only one copy of pg_shadow per cluster, not one per database.

## pg_shadow Columns

Name | Type | References | Description
usename | `name` |  | User name
usesysid | `int4` |  | User id (arbitrary number used to reference this user)
usecreatedb | `bool` |  | User may create databases
usesuper | `bool` |  | User is a superuser
usecatupd | `bool` |  | User may update system catalogs. (Even a superuser may not do this unless this attribute is true.)
passwd | `text` |  | Password
valuntil | `abstime` |  | Account expiry time (only used for password authentication)
useconfig | `text[]` |  | Session defaults for run-time configuration variables

## pg_statistic

pg_statistic stores statistical data about the contents of the database. Entries are created by `ANALYZE` and subsequently used by the query planner. There is one entry for each table column that has been analyzed. Note that all the statistical data is inherently approximate, even assuming that it is up-to-date.

Since different kinds of statistics may be appropriate for different kinds of data, pg_statistic is designed not to assume very much about what sort of statistics it stores. Only extremely general statistics (such as NULL-ness) are given dedicated columns in pg_statistic. Everything else is stored in slots, which are groups of associated columns whose content is identified by a code number in one of the slot's columns. For more information see `src/include/catalog/pg_statistic.h`.

pg_statistic should not be readable by the public, since even statistical information about a table's contents may be considered sensitive. (Example: minimum and maximum values of a salary column might be quite interesting.) pg_stats is a publicly readable view on pg_statistic that only exposes information about those tables that are readable by the current user. pg_stats is also designed to present the information in a more readable format than the underlying pg_statistic table --- at the cost that its schema must be extended whenever new slot types are added.

## pg_statistic Columns

Name | Type | References | Description
starelid | `oid` | pg_class.oid | The table that the described column belongs to
staattnum | `int2` | pg_attribute.attnum | The number of the described column
stanullfrac | `float4` |  | The fraction of the column's entries that are NULL
stawidth | `int4` |  | The average stored width, in bytes, of non-NULL entries
stadistinct | `float4` |  | The number of distinct non-NULL data values in the column. A value greater than zero is the actual number of distinct values. A value less than zero is the negative of a fraction of the number of rows in the table (for example, a column in which values appear about twice on the average could be represented by stadistinct = -0.5). A zero value means the number of distinct values is unknown.
stakindN | `int2` |  | A code number indicating the kind of statistics stored in the Nth slot of the pg_statistic row.
staopN | `oid` | pg_operator.oid | An operator used to derive the statistics stored in the Nth slot. For example, a histogram slot would show the `<` operator that defines the sort order of the data.
stanumbersN | `float4[]` |  | Numerical statistics of the appropriate kind for the Nth slot, or NULL if the slot kind does not involve numerical values.
stavaluesN | `text[]` |  | Column data values of the appropriate kind for the Nth slot, or NULL if the slot kind does not store any data values. For data-type independence, all column data values are converted to external textual form and stored as TEXT datums.

## pg_trigger

This system catalog stores triggers on tables. See under `CREATE TRIGGER` for more information.

## pg_trigger Columns

Name | Type | References | Description
tgrelid | `oid` | pg_class.oid | The table this trigger is on
tgname | `name` |  | Trigger name (must be unique among triggers of same table)
tgfoid | `oid` | pg_proc.oid | The function to be called
tgtype | `int2` |  | Bitmask identifying trigger conditions
tgenabled | `bool` |  | True if trigger is enabled (not presently checked everywhere it should be, so disabling a trigger by setting this false does not work reliably)
tgisconstraint | `bool` |  | True if trigger implements an RI constraint
tgconstrname | `name` |  | RI constraint name
tgconstrrelid | `oid` | pg_class.oid | The table referenced by an RI constraint
tgdeferrable | `bool` |  | True if deferrable
tginitdeferred | `bool` |  | True if initially deferred
tgnargs | `int2` |  | Number of argument strings passed to trigger function
tgattr | `int2vector` |  | Currently unused
tgargs | `bytea` |  | Argument strings to pass to trigger, each null-terminated

pg_class.reltriggers needs to match up with the entries in this table.

## pg_type

This catalog stores information about data types. Scalar types (base types) are created with `CREATE TYPE`. A complex type is automatically created for each table in the database, to represent the row structure of the table. It is also possible to create complex types with `CREATE TYPE AS`, and derived types with `CREATE DOMAIN`.

## pg_type Columns

Name | Type | References | Description
typname | `name` |  | Data type name
typnamespace | `oid` | pg_namespace.oid | The OID of the namespace that contains this type
typowner | `int4` | pg_shadow.usesysid | Owner (creator) of the type
typlen | `int2` |  | For a fixed-size type, typlen is the number of bytes in the internal representation of the type. But for a variable-length type, typlen is negative. -1 indicates a varlena type (one that has a length word), -2 indicates a null-terminated C string.
typbyval | `bool` |  | typbyval determines whether internal routines pass a value of this type by value or by reference. Only `char`, `short`, and `int` equivalent items can be passed by value, so if the type is not 1, 2, or 4 bytes long, PostgreSQL does not have the option of passing by value and so typbyval had better be false. Variable-length types are always passed by reference. Note that typbyval can be false even if the length would allow pass-by-value; this is currently true for type `float4`, for example.
typtype | `char` |  | typtype is `b` for a base type, `c` for a complex type (i.e., a table's row type), `d` for a derived type (i.e., a domain), or `p` for a pseudo-type. See also typrelid and typbasetype.
typisdefined | `bool` |  | True if the type is defined, false if this is a placeholder entry for a not-yet-defined type. When typisdefined is false, nothing except the type name, namespace, and OID can be relied on.
typdelim | `char` |  | Character that separates two values of this type when parsing array input. Note that the delimiter is associated with the array element data type, not the array data type.
typrelid | `oid` | pg_class.oid | If this is a complex type (see typtype), then this field points to the pg_class entry that defines the corresponding table. (For a free-standing composite type, the pg_class entry doesn't really represent a table, but it is needed anyway for the type's pg_attribute entries to link to.) Zero for non-complex types.
typelem | `oid` | pg_type.oid | If typelem is not 0 then it identifies another row in pg_type. The current type can then be subscripted like an array yielding values of type typelem. A true array type is variable length (typlen = -1), but some fixed-length (typlen > 0) types also have nonzero typelem, for example `name` and `oidvector`. If a fixed-length type has a typelem then its internal representation must be N values of the typelem data type with no other data. Variable-length array types have a header defined by the array subroutines.
typinput | `regproc` | pg_proc.oid | Input conversion function
typoutput | `regproc` | pg_proc.oid | Output conversion function
typalign | `char` |  | typalign is the alignment required when storing a value of this type. It applies to storage on disk as well as most representations of the value inside PostgreSQL. When multiple values are stored consecutively, such as in the representation of a complete row on disk, padding is inserted before a datum of this type so that it begins on the specified boundary. The alignment reference is the beginning of the first datum in the sequence. Possible values are: 'c' = CHAR alignment, i.e., no alignment needed. 's' = SHORT alignment (2 bytes on most machines). 'i' = INT alignment (4 bytes on most machines). 'd' = DOUBLE alignment (8 bytes on many machines, but by no means all). For types used in system tables, it is critical that the size and alignment defined in pg_type agree with the way that the compiler will lay out the field in a struct representing a table row.
typstorage | `char` |  | typstorage tells for varlena types (those with typlen = -1) if the type is prepared for toasting and what the default strategy for attributes of this type should be. Possible values are 'p': Value must always be stored plain. 'e': Value can be stored in a secondary relation (if relation has one, see pg_class.reltoastrelid). 'm': Value can be stored compressed inline. 'x': Value can be stored compressed inline or in secondary. Note that 'm' fields can also be moved out to secondary storage, but only as a last resort ('e' and 'x' fields are moved first).
typnotnull | `bool` |  | typnotnull represents a NOT NULL constraint on a type. Presently used for domains only.
typbasetype | `oid` | pg_type.oid | If this is a derived type (see typtype), then typbasetype identifies the type that this one is based on. Zero if not a derived type.
typtypmod | `int4` |  | Domains use typtypmod to record the typmod to be applied to their base type (-1 if base type does not use a typmod). -1 if this type is not a domain.
typndims | `int4` |  | typndims is the number of array dimensions for a domain that is an array (that is, typbasetype is an array type; the domain's typelem will match the base type's typelem). Zero for non-domains and non-array domains.
typdefaultbin | `text` |  | If typdefaultbin is not NULL, it is the nodeToString representation of a default expression for the type. Currently this is only used for domains.
typdefault | `text` |  | typdefault is NULL if the type has no associated default value. If typdefaultbin is not NULL, typdefault must contain a human-readable version of the default expression represented by typdefaultbin. If typdefaultbin is NULL and typdefault is not, then typdefault is the external representation of the type's default value, which may be fed to the type's input converter to produce a constant.