如果表中有某些字段使用TOAST进行存储，那么，通过普通的pg_relation_size('表名')查询不到TOAST字段所占用的空间。如果要查询TOAST字段所占用的空间，可以先查询出TOAST字段对应的OID，再通过pg_relation_size(OID)的方式查询出TOAST字段所占用的空间。
--以下实验验证pg_relation_size('表名')查询不到TOAST字段所占用的空间
testdb=> create table t(id int, remark text);
CREATE TABLE
testdb=> insert into t(id) select random() * 100 from generate_series(1,500);
INSERT 0 500
testdb=> select oid,relname,reltoastrelid from pg_class where relname = 't';
oid | relname | reltoastrelid
-------+---------+---------------
24679 | t | 24682
(1 row)
testdb=> select pg_size_pretty(pg_relation_size(24679));
pg_size_pretty
----------------
24 kB
(1 row)
testdb=> select pg_size_pretty(pg_relation_size(24682));
pg_size_pretty
----------------
0 bytes
(1 row)
TOAST字段所占用的空间现在是0
testdb=> select pg_size_pretty(pg_total_relation_size('t'));
pg_size_pretty
----------------
56 kB
(1 row)
testdb=> insert into t(remark) select repeat(md5(random()::text), 10000) from generate_series(1,1000);
INSERT 0 1000
testdb=> select pg_size_pretty(pg_relation_size(24679));
pg_size_pretty
----------------
72 kB
(1 row)
testdb=> select pg_size_pretty(pg_relation_size(24682));
pg_size_pretty
----------------
4000 kB
(1 row)
TOAST字段所占用的空间已经变为4000kB
testdb=> select pg_size_pretty(pg_total_relation_size('t'));
pg_size_pretty
----------------
4184 kB
(1 row)
使用pg_total_relation_size查出的结果是包括TOAST字段所占用的空间的。
testdb=> create index idx_id_id on t(id);
CREATE INDEX
testdb=> select pg_size_pretty(pg_relation_size(24679));
pg_size_pretty
----------------
72 kB
(1 row)
testdb=> select pg_size_pretty(pg_relation_size(24682));
pg_size_pretty
----------------
4000 kB
(1 row)
testdb=> select pg_size_pretty(pg_total_relation_size('t'));
pg_size_pretty
----------------
4240 kB
(1 row)
增加索引后，OID为24679和24682的大小都不变，而pg_total_relation_size的大小增加了，所以pg_total_relation_size的大小是包括了索引所占用的空间的。
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