PostgreSQL如何获取系统视图/函数对应的源码

Posted 2021-10-08 PostgreSQLChina

作者：吴聪

我们在使用PostgreSQL的时候可能经常会碰到这样的情况，对于某个系统视图的某个字段不知道是干啥的，看了官方文档后也一脸懵逼，不知道是啥意思。

这个时候如果我们可以看到该视图或者该字段对应的源码，那么想要理解起来就会方便多了。

以pg_stat_database这个视图为例，它的两个字段tup_returned和tup_fetched的官方文档解释如下：

• tup_returned：这个数据库中查询返回的行数；
• tup_fetched：这个数据库中查询获取的行数。

看完估计你也和我一样懵逼，返回、获取有啥不同？看这个字面解释压根就不明白啥意思啊，那我们该怎么查看其相关源码呢？

对于这种情况，建议按照下面方式一步步来获取：

1、查看视图对应的SQL语句

pg13@cnndr4pptliot-> psql bill bill -E
psql (13beta3)
Type 'help' for help.
bill@bill=>\\d+ pg_stat_database
                                    View 'pg_catalog.pg_stat_database'
        Column         |           Type           | Collation | Nullable | Default | Storage | Description
-----------------------+--------------------------+-----------+----------+---------+---------+-------------
 datid                 | oid                      |           |          |         | plain   |
 datname               | name                     |           |          |         | plain   |
 numbackends           | integer                  |           |          |         | plain   |
 xact_commit           | bigint                   |           |          |         | plain   |
 xact_rollback         | bigint                   |           |          |         | plain   |
 blks_read             | bigint                   |           |          |         | plain   |
 blks_hit              | bigint                   |           |          |         | plain   |
 tup_returned          | bigint                   |           |          |         | plain   |
 tup_fetched           | bigint                   |           |          |         | plain   |
 tup_inserted          | bigint                   |           |          |         | plain   |
 tup_updated           | bigint                   |           |          |         | plain   |
 tup_deleted           | bigint                   |           |          |         | plain   |
 conflicts             | bigint                   |           |          |         | plain   |
 temp_files            | bigint                   |           |          |         | plain   |
 temp_bytes            | bigint                   |           |          |         | plain   |
 deadlocks             | bigint                   |           |          |         | plain   |
 checksum_failures     | bigint                   |           |          |         | plain   |
 checksum_last_failure | timestamp with time zone |           |          |         | plain   |
 blk_read_time         | double precision         |           |          |         | plain   |
 blk_write_time        | double precision         |           |          |         | plain   |
 stats_reset           | timestamp with time zone |           |          |         | plain   |
View definition:
 SELECT d.oid AS datid,
    d.datname,
        CASE
            WHEN d.oid = 0::oid THEN 0
            ELSE pg_stat_get_db_numbackends(d.oid)
        END AS numbackends,
    pg_stat_get_db_xact_commit(d.oid) AS xact_commit,
    pg_stat_get_db_xact_rollback(d.oid) AS xact_rollback,
    pg_stat_get_db_blocks_fetched(d.oid) - pg_stat_get_db_blocks_hit(d.oid) AS blks_read,
    pg_stat_get_db_blocks_hit(d.oid) AS blks_hit,
    pg_stat_get_db_tuples_returned(d.oid) AS tup_returned,
    pg_stat_get_db_tuples_fetched(d.oid) AS tup_fetched,
    pg_stat_get_db_tuples_inserted(d.oid) AS tup_inserted,
    pg_stat_get_db_tuples_updated(d.oid) AS tup_updated,
    pg_stat_get_db_tuples_deleted(d.oid) AS tup_deleted,
    pg_stat_get_db_conflict_all(d.oid) AS conflicts,
    pg_stat_get_db_temp_files(d.oid) AS temp_files,
    pg_stat_get_db_temp_bytes(d.oid) AS temp_bytes,
    pg_stat_get_db_deadlocks(d.oid) AS deadlocks,
    pg_stat_get_db_checksum_failures(d.oid) AS checksum_failures,
    pg_stat_get_db_checksum_last_failure(d.oid) AS checksum_last_failure,
    pg_stat_get_db_blk_read_time(d.oid) AS blk_read_time,
    pg_stat_get_db_blk_write_time(d.oid) AS blk_write_time,
    pg_stat_get_db_stat_reset_time(d.oid) AS stats_reset
   FROM ( SELECT 0 AS oid,
            NULL::name AS datname
        UNION ALL
         SELECT pg_database.oid,
            pg_database.datname
           FROM pg_database) d;

这样我们就知道了tup_returned和tup_fetched分别对应的是pg_stat_get_db_tuples_returned和pg_stat_get_db_tuples_fetched这两个函数。

2、查看系统函数对应的源码函数

bill@bill=>\\df+ pg_stat_get_db_tuples_returned
List of functions
-[ RECORD 1 ]-------+-----------------------------------------
Schema              | pg_catalog
Name                | pg_stat_get_db_tuples_returned
Result data type    | bigint
Argument data types | oid
Type                | func
Volatility          | stable
Parallel            | restricted
Owner               | postgres
Security            | invoker
Access privileges   |
Language            | internal
Source code         | pg_stat_get_db_tuples_returned
Description         | statistics: tuples returned for database

以pg_stat_get_db_tuples_returned为例，其对应的源码函数是pg_stat_get_db_tuples_returned。

3、分析相关源码

既然知道了对应的源码函数那就简单了，直接去搜索就完事了。

Datum
pg_stat_get_tuples_returned(PG_FUNCTION_ARGS)
{
    Oid            relid = PG_GETARG_OID(0);
    int64        result;
    PgStat_StatTabEntry *tabentry;
    if ((tabentry = pgstat_fetch_stat_tabentry(relid)) == NULL)
        result = 0;
    else
        result = (int64) (tabentry->tuples_returned);
    PG_RETURN_INT64(result);
}

接着慢慢分析相关的源码，查看相关注释就可以了，我们可以得出结论：

• tuples_returned,：直接扫描表的tuple行数 + 直接扫描索引的索引条目数。
• tuples_fetched：通过索引指向的tid去回表访问的tuple行数。

参考链接：
http://www.postgres.cn/docs/13/monitoring-stats.html#MONITORING-PG-STAT-DATABASE-VIEW