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。
原创文章，作者：奋斗，如若转载，请注明出处：https://blog.ytso.com/237305.html