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.
The name (optionally schema-qualified) of a table to be truncated.
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.
Refuse to truncate if any of the tables have foreign-key references from tables that are not to be truncated. This is the default.
Only the owner of a table can TRUNCATE it.
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 run any ON DELETE triggers that might exist for the tables.
| Warning |
TRUNCATE is not MVCC-safe (see Chapter 13 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 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, however: the truncation will be safely rolled back if the surrounding transaction does not commit. |