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How can I speed up this function?

From: David Mitchell <david(dot)mitchell(at)telogis(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: How can I speed up this function?
Date: 2005-06-28 02:37:34
Message-ID: 42C0B7EE.4040608@telogis.com (view raw or flat)
Thread:
Lists: pgsql-performance
We have the following function in our home grown mirroring package, but 
it isn't running as fast as we would like. We need to select statements 
from the pending_statement table, and we want to select all the 
statements for a single transaction (pending_trans) in one go (that is, 
we either select all the statements for a transaction, or none of them). 
We select as many blocks of statements as it takes to top the 100 
statement limit (so if the last transaction we pull has enough 
statements to put our count at 110, we'll still take it, but then we're 
done).

Here is our function:

CREATE OR REPLACE FUNCTION dbmirror.get_pending()
   RETURNS SETOF dbmirror.pending_statement AS
$BODY$

DECLARE
         count INT4;
         transaction RECORD;
         statement dbmirror.pending_statement;
     BEGIN
         count := 0;

         FOR transaction IN SELECT t.trans_id as ID
         FROM pending_trans AS t WHERE fetched = false
         ORDER BY trans_id LIMIT 50
     LOOP
             update pending_trans set fetched =  true where trans_id = 
transaction.id;

	    FOR statement IN SELECT s.id, s.transaction_id, s.table_name, s.op, 
s.data
                 FROM dbmirror.pending_statement AS s
                 WHERE s.transaction_id = transaction.id
                 ORDER BY s.id ASC
             LOOP
                 count := count + 1;

                 RETURN NEXT statement;
             END LOOP;

             IF count > 100 THEN
                 EXIT;
             END IF;
         END LOOP;

         RETURN;
     END;$BODY$
   LANGUAGE 'plpgsql' VOLATILE;

Table Schemas:

CREATE TABLE dbmirror.pending_trans
(
   trans_id oid NOT NULL,
   fetched bool DEFAULT false,
   CONSTRAINT pending_trans_pkey PRIMARY KEY (trans_id)
)
WITHOUT OIDS;

CREATE TABLE dbmirror.pending_statement
(
   id oid NOT NULL DEFAULT nextval('dbmirror.statement_id_seq'::text),
   transaction_id oid NOT NULL,
   table_name text NOT NULL,
   op char NOT NULL,
   data text NOT NULL,
   CONSTRAINT pending_statement_pkey PRIMARY KEY (id)
)
WITHOUT OIDS;

CREATE UNIQUE INDEX idx_stmt_tran_id_id
   ON dbmirror.pending_statement
   USING btree
   (transaction_id, id);

Postgres 8.0.1 on Linux.

Any Help would be greatly appreciated.

Regards

-- 
David Mitchell
Software Engineer
Telogis

Responses

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Next:From: Michael FuhrDate: 2005-06-28 02:49:18
Subject: Re: Performance analysis of plpgsql code
Previous:From: Karl O. PincDate: 2005-06-28 02:36:51
Subject: Re: Poor index choice -- multiple indexes of the same

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