PostgreSQL truncate table会释放索引的空间

apple=# create table test(id integer, info text);
CREATE TABLE
apple=# insert into test select generate_series(1, 100000), md5('test');
INSERT 0 100000
apple=# select * from test limit 1;
 id |               info
----+----------------------------------
  1 | 098f6bcd4621d373cade4e832627b4f6
(1 row)

apple=# select pg_size_pretty(pg_relation_size('test'));
 pg_size_pretty
----------------
 6672 kB
(1 row)

apple=# create index idx_test_id on test(id);
CREATE INDEX
apple=# select * from pg_indexes_size('test');
 pg_indexes_size
-----------------
         2260992
(1 row)

apple=# select * from pg_size_pretty(pg_indexes_size('test'));
 pg_size_pretty
----------------
 2208 kB
(1 row)

apple=# truncate test;
TRUNCATE TABLE
apple=# select * from pg_size_pretty(pg_indexes_size('test'));
 pg_size_pretty
----------------
 8192 bytes
(1 row)

execute query success, query is select * from pg_size_pretty(pg_indexes_size('test'));
apple=# select pg_size_pretty(pg_relation_size('test'));
 pg_size_pretty
----------------
 0 bytes
(1 row)

1.delete，表占用空间不变，这个众所周知，所谓的water-mark不变；表上索引占用空间也不会变；

2.默认不会重置表对应的sequence，除非在后面加上 RESTART IDENTITY。

3.truncate table tb_name，表与表上索引的空间占用均回到建立索引或表时的initial参数。

4.定期rebuild索引是一个好习惯，一是提高索引效率二是释放存贮区。

5.cascade慎用。

TRUNCATE

TRUNCATE — empty a table or set of tables

Synopsis

TRUNCATE [ TABLE ] [ ONLY ] name [ * ] [, ... ]
    [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
Description

TRUNCATE quickly removes all rows from a set of tables. It has the same effect as an unqualified DELETE on each table, but since it does not actually scan the tables it is faster. Furthermore, it reclaims disk space immediately, rather than requiring a subsequent VACUUM operation. This is most useful on large tables.

Parameters

name
The name (optionally schema-qualified) of a table to truncate. If ONLY is specified before the table name, only that table is truncated. If ONLY is not specified, the table and all its descendant tables (if any) are truncated. Optionally, * can be specified after the table name to explicitly indicate that descendant tables are included.

RESTART IDENTITY
Automatically restart sequences owned by columns of the truncated table(s).

CONTINUE IDENTITY
Do not change the values of sequences. This is the default.

CASCADE
Automatically truncate all tables that have foreign-key references to any of the named tables, or to any tables added to the group due to CASCADE.

RESTRICT
Refuse to truncate if any of the tables have foreign-key references from tables that are not listed in the command. This is the default.

Notes

You must have the TRUNCATE privilege on a table to truncate it.

TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates on, which blocks all other concurrent operations on the table. When RESTART IDENTITY is specified, any sequences that are to be restarted are likewise locked exclusively. If concurrent access to a table is required, then the DELETE command should be used instead.

TRUNCATE cannot be used on a table that has foreign-key references from other tables, unless all such tables are also truncated in the same command. Checking validity in such cases would require table scans, and the whole point is not to do one. The CASCADE option can be used to automatically include all dependent tables — but be very careful when using this option, or else you might lose data you did not intend to!

TRUNCATE will not fire any ON DELETE triggers that might exist for the tables. But it will fire ON TRUNCATE triggers. If ON TRUNCATE triggers are defined for any of the tables, then all BEFORE TRUNCATE triggers are fired before any truncation happens, and all AFTER TRUNCATE triggers are fired after the last truncation is performed and any sequences are reset. The triggers will fire in the order that the tables are to be processed (first those listed in the command, and then any that were added due to cascading).

TRUNCATE is not MVCC-safe. After truncation, the table will appear empty to concurrent transactions, if they are using a snapshot taken before the truncation occurred. See Section 13.5 for more details.

TRUNCATE is transaction-safe with respect to the data in the tables: the truncation will be safely rolled back if the surrounding transaction does not commit.

When RESTART IDENTITY is specified, the implied ALTER SEQUENCE RESTART operations are also done transactionally; that is, they will be rolled back if the surrounding transaction does not commit. This is unlike the normal behavior of ALTER SEQUENCE RESTART. Be aware that if any additional sequence operations are done on the restarted sequences before the transaction rolls back, the effects of these operations on the sequences will be rolled back, but not their effects on currval(); that is, after the transaction currval() will continue to reflect the last sequence value obtained inside the failed transaction, even though the sequence itself may no longer be consistent with that. This is similar to the usual behavior of currval() after a failed transaction.

TRUNCATE is not currently supported for foreign tables. This implies that if a specified table has any descendant tables that are foreign, the command will fail.