Re: Delete rule does not prevent truncate - Mailing list pgsql-general

On 07/22/2015 06:13 AM, Tim Smith wrote:
> Melvin,
>
> May I point out that the manual states :
> "TRUNCATE quickly removes all rows from a set of tables. It has the same
> effect as an unqualified DELETE on each table"
>
> Thus, if you are telling me to effectively think of TRUNCATE as an alias
> to DELETE, then I would think its not entirely unreasonable of me to
> expect a rule preventing DELETE to also cover truncate, since the rule
> would no doubt prevent an unqualified DELETE, would it not ?!?

If  you go further down into the Notes section you find:

"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).
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 when you want fast over safety.

>
> On 22 July 2015 at 14:03, Melvin Davidson <melvin6925@gmail.com
> <mailto:melvin6925@gmail.com>> wrote:
>
>     Actually, if you use a TRIGGER instead of rule, you can handle this.
>     The manual states event can be:
>
>     INSERT
>     UPDATE [ OFcolumn_name  [, ... ] ]
>     DELETE
>     *TRUNCATE <-----*
>
>     http://www.postgresql.org/docs/9.4/interactive/sql-createtrigger.html
>
>     I suggest you review carefully.
>
>     On Wed, Jul 22, 2015 at 8:53 AM, Tim Smith
>     <randomdev4+postgres@gmail.com
>     <mailto:randomdev4+postgres@gmail.com>> wrote:
>
>         Hi,
>
>         I very much hope this is an accidental bug rather than a
>         deliberate feature !
>
>         PostgreSQL 9.4.4
>
>         create rule no_auditupd as on update to app_security.app_audit do
>         instead nothing;
>         create rule no_auditdel as on delete to app_security.app_audit do
>         instead nothing;
>
>         \d+  app_security.app_audit
>         <snip>
>         Rules:
>              no_auditdel AS
>              ON DELETE TO app_security.app_audit DO INSTEAD NOTHING
>              no_auditupd AS
>              ON UPDATE TO app_security.app_audit DO INSTEAD NOTHING
>
>         The truncate trashes the whole table  ;-(
>
>         According to the FabulousManual(TM) :
>         event : The event is one of SELECT, INSERT, UPDATE, or DELETE.
>
>         Thus I can't create a rule to "do nothing" on truncates, thus I
>         am stuck !
>
>
>         --
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>
>
>
>
>     --
>     *Melvin Davidson*
>     I reserve the right to fantasize.  Whether or not you
>     wish to share my fantasy is entirely up to you.
>
>


--
Adrian Klaver
adrian.klaver@aklaver.com