如何找到聚合 PostgreSQL 函数的实现？

Question

我对 stddev_pop() 函数的实现或代码感兴趣。我理解它的作用，我知道文档中有定义，但我对代码本身感兴趣。 我试图在 pgAdmin 的公共模式/函数和聚合中找到它，但它不在那里。我也尝试过这个查询：

SELECT *
FROM pg_proc
WHERE proname = 'stddev_pop';

这给了我 6 个不同的结果。但是当我尝试使用函数的 oid 进行查询时，会发生以下情况：

SELECT pg_get_functiondef(2724);
ERROR:  "stddev_pop" is an aggregate function
SQL state: 42809

也在 psql 中我尝试过：

isa=# \df stddev_pop
                            List of functions
   Schema   |    Name    | Result data type | Argument data types | Type
------------+------------+------------------+---------------------+------
 pg_catalog | stddev_pop | numeric          | bigint              | agg
 pg_catalog | stddev_pop | double precision | double precision    | agg
 pg_catalog | stddev_pop | numeric          | integer             | agg
 pg_catalog | stddev_pop | numeric          | numeric             | agg
 pg_catalog | stddev_pop | double precision | real                | agg
 pg_catalog | stddev_pop | numeric          | smallint            | agg
(6 rows)

但无法更进一步。

如何“查看”聚合函数？

Answer 1

您要查询

pg_aggregate

目录：

SELECT a.aggfnoid::regprocedure AS aggregate,
       a.agginitval AS initial_value,
       a.aggtransfn::regprocedure AS state_transition_function,
       a.aggfinalfn::regprocedure AS final_function
FROM pg_aggregate AS a
   JOIN pg_proc AS p
      ON a.aggfnoid = p.oid
WHERE p.prokind = 'a'
  AND p.proname = 'stddev_pop';

          aggregate           │ initial_value │             state_transition_function             │            final_function             
══════════════════════════════╪═══════════════╪═══════════════════════════════════════════════════╪═══════════════════════════════════════
 stddev_pop(bigint)           │ ∅             │ int8_accum(internal,bigint)                       │ numeric_stddev_pop(internal)
 stddev_pop(integer)          │ ∅             │ int4_accum(internal,integer)                      │ numeric_poly_stddev_pop(internal)
 stddev_pop(smallint)         │ ∅             │ int2_accum(internal,smallint)                     │ numeric_poly_stddev_pop(internal)
 stddev_pop(real)             │ {0,0,0}       │ float4_accum(double precision[],real)             │ float8_stddev_pop(double precision[])
 stddev_pop(double precision) │ {0,0,0}       │ float8_accum(double precision[],double precision) │ float8_stddev_pop(double precision[])
 stddev_pop(numeric)          │ ∅             │ numeric_accum(internal,numeric)                   │ numeric_stddev_pop(internal)
(6 rows)

Answer 2

这是一个内置的聚合函数。您将在 postgres 代码库中找到实现。

Datum
float8_stddev_pop(PG_FUNCTION_ARGS)
{
    ArrayType  *transarray = PG_GETARG_ARRAYTYPE_P(0);
    float8     *transvalues;
    float8      N,
                Sxx;

    transvalues = check_float8_array(transarray, "float8_stddev_pop", 3);
    N = transvalues[0];
    /* ignore Sx */
    Sxx = transvalues[2];

    /* Population stddev is undefined when N is 0, so return NULL */
    if (N == 0.0)
        PG_RETURN_NULL();

    /* Note that Sxx is guaranteed to be non-negative */

    PG_RETURN_FLOAT8(sqrt(Sxx / N));
}