TRUNCATE(7) SQL Commands TRUNCATE(7)NAME
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 trun‐
cated table(s).
CONTINUE IDENTITY
Do not change the values of sequences. This is the default.
CASCADE
Automatically truncate all tables that have foreign-key refer‐
ences 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 refer‐
ences 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. If con‐
current 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 trig‐
gers 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).
Warning: TRUNCATE is not MVCC-safe (see in the documentation 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 SHARE lock, which
would block TRUNCATE until that transaction completes. So trun‐
cation 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 trun‐
cated 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 transac‐
tion does not commit.
Warning: Any ALTER SEQUENCE RESTART operations performed as a
consequence of using the RESTART IDENTITY option are nontransac‐
tional 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 TRUN‐
CATE is performed inside a transaction block that is aborted
afterwards. For example, consider
BEGIN;
TRUNCATE TABLE foo RESTART IDENTITY;
COPY foo FROM ...;
COMMIT;
If the COPY fails 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.
EXAMPLES
Truncate the tables bigtable and fattable:
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 that reference
othertable via foreign-key constraints:
TRUNCATE othertable CASCADE;
COMPATIBILITY
The SQL:2008 standard includes a TRUNCATE command with the syntax TRUN‐
CATE TABLE tablename. The clauses CONTINUE IDENTITY/RESTART IDENTITY
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.
SQL - Language Statements 2013-04-02 TRUNCATE(7)
