|The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 488 pages
RRP £14.95 ($19.95)
Sales of this book support the PostgreSQL project! Get a printed copy>>>
TRUNCATE -- empty a table or set of tables
TRUNCATE [ TABLE ] [ ONLY ] name [, ... ] [ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
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
operation. This is most useful on large tables.
The name (optionally schema-qualified) of a table to be
ONLYis specified, only that table is truncated. If
ONLYis not specified, the table and all its descendant tables (if any) are truncated.
- Automatically restart sequences owned by columns of the truncated table(s).
- Do not change the values of sequences. This is the default.
Automatically truncate all tables that have foreign-key references
to any of the named tables, or to any tables added to the group
- 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.
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. If concurrent access to a table is required, then
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
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
triggers that might exist for the tables. But it will fire
ON TRUNCATE triggers.
ON TRUNCATE triggers are defined for any of
the tables, then all
BEFORE TRUNCATE triggers are
fired before any truncation happens, and all
TRUNCATE triggers are fired after the last truncation is
performed. 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).
TRUNCATEis not MVCC-safe (see Volume 1A: 11 Concurrency Control for general information about MVCC). After truncation, the table will appear empty to all concurrent transactions, even if they are using a snapshot taken before the truncation occurred. This will only be an issue for a transaction that did not access the truncated table before the truncation happened--any transaction that has done so would hold at least an
ACCESS SHARElock, which would block
TRUNCATEuntil that transaction completes. So truncation will not cause any apparent inconsistency in the table contents for successive queries on the same table, but it could cause visible inconsistency between the contents of the truncated table and other tables in the database.
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.
ALTER SEQUENCE RESTARToperations performed as a consequence of using the
RESTART IDENTITYoption are nontransactional and will not be rolled back on failure. To minimize the risk, these operations are performed only after all the rest of
TRUNCATE's work is done. However, there is still a risk if
TRUNCATEis performed inside a transaction block that is aborted afterwards. For example, considerBEGIN; TRUNCATE TABLE foo RESTART IDENTITY; COPY foo FROM ...; COMMIT;
COPYfails partway through, the table data rolls back correctly, but the sequences will be left with values that are probably smaller than they had before, possibly leading to duplicate-key failures or other problems in later transactions. If this is likely to be a problem, it's best to avoid using
RESTART IDENTITY, and accept that the new contents of the table will have higher serial numbers than the old.
Truncate the tables
TRUNCATE bigtable, fattable;
The same, and also reset any associated sequence generators:
TRUNCATE bigtable, fattable RESTART IDENTITY;
Truncate the table
othertable, and cascade to any tables
othertable via foreign-key
TRUNCATE othertable CASCADE;
The SQL:2008 standard includes a
TRUNCATE command with the syntax
TRUNCATE TABLE tablename.
also appear in that standard but have slightly different but related meanings.
Some of the concurrency behavior of this command is left implementation-defined
by the standard, so the above notes should be considered and compared with
other implementations if necessary.
|ISBN 9781906966058||The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference||See the print edition|