作者：吴聪
我们在使用PostgreSQL的时候大概经常会碰到如许的情况，对于某个体系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的官方文档表明如下：
- 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、获取有啥差别？看这个字面表明压根就不明确啥意思啊，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 -Epsql (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;
如许我们就知道了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、查看体系函数对应的源码函数
bill@bill=>\df+ pg_stat_get_db_tuples_returnedList of functions-[ RECORD 1 ]-------+-----------------------------------------Schema | pg_catalogName | pg_stat_get_db_tuples_returnedResult data type | bigintArgument data types | oidType | funcVolatility | stableParallel | restrictedOwner | postgresSecurity | invokerAccess privileges |Language | internalSource code | pg_stat_get_db_tuples_returnedDescription | 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。
Datumpg_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);}
接着逐步分析相关的源码，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 |
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