> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/15/internals/system-catalogs/pg_aggregate.md).

# 53.2. pg\_aggregate

## 53.2. pg\_aggregate

The catalog `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`](/15/internals/system-catalogs/pg_proc.md). The `pg_proc` entry carries the aggregate's name, input and output data types, and other information that is similar to ordinary functions.

**Table 53.2. `pg_aggregate` Columns**

| <p>Column Type</p><p>Description</p>                                                                                                                                                                                                                                                                                                                                                                       |
| ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| <p><code>aggfnoid</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p><code>pg\_proc</code> OID of the aggregate function</p>                                                                                                                                                                                     |
| <p><code>aggkind</code> <code>char</code></p><p>Aggregate kind: <code>n</code> for “normal” aggregates, <code>o</code> for “ordered-set” aggregates, or <code>h</code> for “hypothetical-set” aggregates</p>                                                                                                                                                                                               |
| <p><code>aggnumdirectargs</code> <code>int2</code></p><p>Number of direct (non-aggregated) arguments of an ordered-set or hypothetical-set aggregate, counting a variadic array as one argument. If equal to <code>pronargs</code>, the aggregate must be variadic and the variadic array describes the aggregated arguments as well as the final direct arguments. Always zero for normal aggregates.</p> |
| <p><code>aggtransfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Transition function</p>                                                                                                                                                                                                                   |
| <p><code>aggfinalfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Final function (zero if none)</p>                                                                                                                                                                                                         |
| <p><code>aggcombinefn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Combine function (zero if none)</p>                                                                                                                                                                                                     |
| <p><code>aggserialfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Serialization function (zero if none)</p>                                                                                                                                                                                                |
| <p><code>aggdeserialfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Deserialization function (zero if none)</p>                                                                                                                                                                                            |
| <p><code>aggmtransfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Forward transition function for moving-aggregate mode (zero if none)</p>                                                                                                                                                                 |
| <p><code>aggminvtransfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Inverse transition function for moving-aggregate mode (zero if none)</p>                                                                                                                                                              |
| <p><code>aggmfinalfn</code> <code>regproc</code> (references <a href="/15/internals/system-catalogs/pg_proc.md"><code>pg\_proc</code></a>.<code>oid</code>)</p><p>Final function for moving-aggregate mode (zero if none)</p>                                                                                                                                                                              |
| <p><code>aggfinalextra</code> <code>bool</code></p><p>True to pass extra dummy arguments to <code>aggfinalfn</code></p>                                                                                                                                                                                                                                                                                    |
| <p><code>aggmfinalextra</code> <code>bool</code></p><p>True to pass extra dummy arguments to <code>aggmfinalfn</code></p>                                                                                                                                                                                                                                                                                  |
| <p><code>aggfinalmodify</code> <code>char</code></p><p>Whether <code>aggfinalfn</code> modifies the transition state value: <code>r</code> if it is read-only, <code>s</code> if the <code>aggtransfn</code> cannot be applied after the <code>aggfinalfn</code>, or <code>w</code> if it writes on the value</p>                                                                                          |
| <p><code>aggmfinalmodify</code> <code>char</code></p><p>Like <code>aggfinalmodify</code>, but for the <code>aggmfinalfn</code></p>                                                                                                                                                                                                                                                                         |
| <p><code>aggsortop</code> <code>oid</code> (references <a href="/15/internals/system-catalogs/pg_operator.md"><code>pg\_operator</code></a>.<code>oid</code>)</p><p>Associated sort operator (zero if none)</p>                                                                                                                                                                                            |
| <p><code>aggtranstype</code> <code>oid</code> (references <a href="/15/internals/system-catalogs/pg_type.md"><code>pg\_type</code></a>.<code>oid</code>)</p><p>Data type of the aggregate function's internal transition (state) data</p>                                                                                                                                                                  |
| <p><code>aggtransspace</code> <code>int4</code></p><p>Approximate average size (in bytes) of the transition state data, or zero to use a default estimate</p>                                                                                                                                                                                                                                              |
| <p><code>aggmtranstype</code> <code>oid</code> (references <a href="/15/internals/system-catalogs/pg_type.md"><code>pg\_type</code></a>.<code>oid</code>)</p><p>Data type of the aggregate function's internal transition (state) data for moving-aggregate mode (zero if none)</p>                                                                                                                        |
| <p><code>aggmtransspace</code> <code>int4</code></p><p>Approximate average size (in bytes) of the transition state data for moving-aggregate mode, or zero to use a default estimate</p>                                                                                                                                                                                                                   |
| <p><code>agginitval</code> <code>text</code></p><p>The initial value of the transition state. This is a text field containing the initial value in its external string representation. If this field is null, the transition state value starts out null.</p>                                                                                                                                              |
| <p><code>aggminitval</code> <code>text</code></p><p>The initial value of the transition state for moving-aggregate mode. This is a text field containing the initial value in its external string representation. If this field is null, the transition state value starts out null.</p>                                                                                                                   |

New aggregate functions are registered with the [`CREATE AGGREGATE`](/15/reference/sql-commands/create-aggregate.md) command. See [Section 38.12](/15/server-programming/extending-sql/user-defined-aggregates.md) for more information about writing aggregate functions and the meaning of the transition functions, etc.

***

原文：[PostgreSQL 15.19 Documentation](/15/internals/system-catalogs/pg_aggregate.md)（英文原文，待翻譯）


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