Postgresql 中文操作指南

53.2. pg_aggregate #

目录 pg_aggregate 存储有关聚合函数的信息。聚合函数是在一组值（通常是与查询条件相匹配的每行的一列）上操作的函数，并返回从所有这些值计算出的单个值。典型的聚合函数是 sum 、 count 和 max 。 pg_aggregate 中的每个条目都是 pg_proc 中某个条目的扩展。 pg_proc 条目承载聚合的名称、输入和输出数据类型，以及与普通函数类似的其他信息。

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. 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

Column Type

Description

aggfnoid regproc (references pg_proc.oid)

pg_proc OID of the aggregate function

aggkind char

Aggregate kind: n for “normal” aggregates, o for “ordered-set” aggregates, or h for “hypothetical-set” aggregates

aggnumdirectargs int2

Number of direct (non-aggregated) arguments of an ordered-set or hypothetical-set aggregate, counting a variadic array as one argument. If equal to pronargs, 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.

aggtransfn regproc (references pg_proc.oid)

Transition function

aggfinalfn regproc (references pg_proc.oid)

Final function (zero if none)

aggcombinefn regproc (references pg_proc.oid)

Combine function (zero if none)

aggserialfn regproc (references pg_proc.oid)

Serialization function (zero if none)

aggdeserialfn regproc (references pg_proc.oid)

Deserialization function (zero if none)

aggmtransfn regproc (references pg_proc.oid)

Forward transition function for moving-aggregate mode (zero if none)

aggminvtransfn regproc (references pg_proc.oid)

Inverse transition function for moving-aggregate mode (zero if none)

aggmfinalfn regproc (references pg_proc.oid)

Final function for moving-aggregate mode (zero if none)

aggfinalextra bool

True to pass extra dummy arguments to aggfinalfn

aggmfinalextra bool

True to pass extra dummy arguments to aggmfinalfn

aggfinalmodify char

Whether aggfinalfn modifies the transition state value: r if it is read-only, s if the aggtransfn cannot be applied after the aggfinalfn, or w if it writes on the value

aggmfinalmodify char

Like aggfinalmodify, but for the aggmfinalfn

aggsortop oid (references pg_operator.oid)

Associated sort operator (zero if none)

aggtranstype oid (references pg_type.oid)

Data type of the aggregate function’s internal transition (state) data

aggtransspace int4

Approximate average size (in bytes) of the transition state data, or zero to use a default estimate

aggmtranstype oid (references pg_type.oid)

Data type of the aggregate function’s internal transition (state) data for moving-aggregate mode (zero if none)

aggmtransspace int4

Approximate average size (in bytes) of the transition state data for moving-aggregate mode, or zero to use a default estimate

agginitval text

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.

aggminitval text

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.

使用 CREATE AGGREGATE 命令注册新聚合函数。有关编写聚合函数、过渡函数含义等的更多信息，请参阅 Section 38.12 。

New aggregate functions are registered with the CREATE AGGREGATE command. See Section 38.12 for more information about writing aggregate functions and the meaning of the transition functions, etc.