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Re: Delete rule does not prevent truncate

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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@xxxxxxxxx
<mailto:melvin6925@xxxxxxxxx>> 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@xxxxxxxxx
    <mailto:randomdev4+postgres@xxxxxxxxx>> 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@xxxxxxxxxxx


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