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From: Josh Berkus <josh@agliodbs.com>
Reply-To: josh@agliodbs.com
Organization: Aglio Database Solutions
To: "Beth Gatewood" <beth@vizxlabs.com>, <pgsql-sql@postgresql.org>
Subject: Re: writing a function to mimic mysql last_insert_id
Date: Wed, 11 Sep 2002 10:53:49 -0700
References: <000901c259ba$c5b7ee70$0c00000a@bethvizx>
In-Reply-To: <000901c259ba$c5b7ee70$0c00000a@bethvizx>
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Message-Id: <200209111053.49837.josh@agliodbs.com>


Beth,

> However, we are in the midst of porting a mysql database to pgsql.  The p=
erl
> application that is tied to this mysql database heavily utilizes
> last_insert_id.  In an effort to save the perl developers the pain of hav=
ing
> to go in and modify a bunch of their SQL to call out to seq.currval, I
> thought I would attempt to mimic this function.
>=20
> Perhaps there is another way to approach this than writing this type of
> function?

Not really.   CURRVAL requires a sequence name as a parameter because, in=
=20
Postgres, one can have more than one sequence for a table.   Further, in=20
Postgres, you can update more than one table at a time using a function,=20
trigger, or rule; how would any function know which sequence id to return?

While it would be relatively easy for a function to look up what seqeunces=
=20
were related to a table in the pg_* tables, you are left with the difficult=
=20
task of determining what statement the user last ran.  This would, I think,=
=20
involve hacking MVCC to some degree.

If you're serious about pursuing this, I suggest posting your question on=
=20
PGSQL-HACKERS to get help with hacking MVCC to determine a connection's las=
t=20
action.  I have no idea how easy or difficult this would be.

A second possibility would be writing triggers for all tables that place a=
=20
value into a temporary table that can be read back by your custom function.

You may find it less work, however, to do a search-and-replace on calls to=
=20
last_inser_id().   But if you do write a replacement function, please post =
it=20
on TechDocs!

--=20
-Josh Berkus
 Aglio Database Solutions
 San Francisco