Re: Truncate if exists - Mailing list pgsql-hackers

<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">>For starters, the use-case hasn't been explained to my satisfaction.<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> In
whatsituation is it actually helpful to TRUNCATE a table that's<p style=" margin-top:0px; margin-bottom:0px;
margin-left:0px;margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> not there yet? Aren't
yougoing to have to do a CREATE IF NOT EXISTS<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> to keep from failing later in the script?
Ifso, why not just do that<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px;
-qt-block-indent:0;text-indent:0px; -qt-user-state:0;">> first?<p style="-qt-paragraph-type:empty; margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> <p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">There is a
usecase for the truncate 'mutliple' tables, maybe less clear for a single table.<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">Sébastien
willspeak here I suppose.<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; "> <p style=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> Second, to my mind the point of a
multi-tableTRUNCATE is to ensure that<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px;
-qt-block-indent:0;text-indent:0px; -qt-user-state:0;">> all the referenced tables get reset to empty *together*.
Withsomething<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0;
text-indent:0px;-qt-user-state:0;">> like this, you'd have no such guarantee. Consider a timeline like this:<p
style="margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">><p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px;
-qt-block-indent:0;text-indent:0px; -qt-user-state:0;">> Session 1 Session 2<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> <p
style="margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">>TRUNCATE IF EXISTS a, b, c;<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> ... finds c doesn't exist ...<p style="
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">>... working on a and b ...<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> CREATE TABLE c ( ... );<p style="
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">>INSERT INTO c ...;<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> ... commits ...<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> <p
style="margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">>Now we have a, b, and c, but c isn't empty, violating the expectations<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> of
session1. So even if there's a use-case for IF EXISTS on a single<p style=" margin-top:0px; margin-bottom:0px;
margin-left:0px;margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;">> table, I think it's very
verydubious to allow it in multi-table<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px;
-qt-block-indent:0;text-indent:0px; -qt-user-state:0;">> commands.<p style="-qt-paragraph-type:empty;
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> <p style="
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">well,in such case you probably don't want to use IF EXISTS.<p style="-qt-paragraph-type:empty;
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; "> <p style="
margin-top:0px;margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">--<p style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px;
-qt-block-indent:0;text-indent:0px; -qt-user-state:0;">Cédric Villemain +33 (0)6 20 30 22 52<p style=" margin-top:0px;
margin-bottom:0px;margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;
-qt-user-state:0;">http://2ndQuadrant.fr/<pstyle=" margin-top:0px; margin-bottom:0px; margin-left:0px;
margin-right:0px;-qt-block-indent:0; text-indent:0px; -qt-user-state:0;">PostgreSQL: Support 24x7 - Développement,
Expertiseet Formation