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Re: Patch for gdl2 to handle LIMIT PostgreSQL keyword in PostgreSQLAdapt


From: David Ayers
Subject: Re: Patch for gdl2 to handle LIMIT PostgreSQL keyword in PostgreSQLAdaptor.
Date: Mon, 16 Mar 2009 11:49:21 +0100

Thanks a lot Thierry,

Am Sonntag, den 15.03.2009, 22:27 +0100 schrieb Thierry DELHAISE:

> Please find enclosed a patch that implement/use the PosgreSQL "LIMIT"
> keyword in PostgreSQLAdaptor.

I'm not yet convinced that we want to add this.  The
EOFetchSpecification documents fetch limit to /either/ stop fetching
objects when the limit is reached /or/ prompt the user to continue
fetching.

Now most database LIMIT implementation I know will merely return a
result until limit is reached and have no way of "continuing" retrieval.
What we do often have (and this includes PostgreSQL) is OFFSET to act
like we are continuing in a subsequent query, but any inserts/deletes or
updates to the sort order attributes could falsify this result.

OTOH, I assume that for large result sets, this could be a nasty issue
if the database needs to prepare the full result set. (GDL2 should only
be requesting the data up to the fetch limit though.  If that's not
happening please file a bug.)

So are you requesting this for sake of performance?
or because of "correctness" due to SQL-statement analysis?

If it's the latter, then I'd rather ponder a bit more.  If it's the
former, then please open a bug report and I'll see if we are doing more
than we should when we have a fetch limit set and whether we can
optimize that first.

Cheers,
David






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