Re: Am I locking more than I need to?

[Mike Rylander <miker@purplefrog.com>]

On Friday 21 May 2004 06:24 pm, Jeff Davis wrote:
> On Fri, 2004-05-21 at 14:33, Carl E. McMillin wrote:
> > Scenario:
> >
> > SELECT ... WHERE cart_id=X FOR UPDATE
> >
> > IF (NOT FOUND) THEN
> >   BEGIN
> > --Here is where nothing is locked.
> > --No way to guarantee no one else will create a record before we do.
> >   INSERT ...
> >   END;
> > END IF;
>
> Instead, I was thinking more like:
>
> BEGIN
> SELECT ... WHERE cart_id=X FOR UPDATE
> IF (NOT FOUND) THEN
> --Here is where nothing is locked.
> --No way to guarantee no one else will create a record before we do.
>   INSERT ...
> ELSE
>   UPDATE ...
> END IF;
> END;

This is basically what I am doing.  See below for the PL/PGSQL for a
trigger based implimentation.  It effectively SERIALIZEs the one table
in question, any other table perfrom at the normail speed.  

Hope it helps!

-miker  (see below)

-----------------------------------------


-- 
-- Merge on INSERT functionallity for Postgres 7.3+
-- 
-- miker@purplefrog.com / 5-1-04
-- 
-- CAVEAT EMPTOR: Uses table locks to avoid concurrency issues,
-- 		  so it WILL slow down heavily loaded tables.
-- 		  This effecivly puts the table into
-- 		  TRANSACTION ISOLATION LEVEL SERIALIZABLE mode.
-- 

CREATE OR REPLACE FUNCTION add_merge_on_insert (
	TEXT,  -- table name
	TEXT,  -- key column
	TEXT[] -- column list to update on deduplication
 ) RETURNS TEXT
   RETURNS NULL ON NULL INPUT
   SECURITY INVOKER
   LANGUAGE 'plpgsql'
   AS '

DECLARE
	tablename	ALIAS FOR $1;
	keycol		ALIAS FOR $2;
	updatecols	ALIAS FOR $3;
	trig		TEXT;
	arraydims	TEXT;

BEGIN
	trig := \'
		CREATE FUNCTION "\' || tablename || \'_merge_on_insert_proc" () RETURNS TRIGGER AS \'\'
		DECLARE
			orig \' || quote_ident(tablename) || \'%ROWTYPE;
		BEGIN
			LOCK TABLE \' || quote_ident(tablename) || \' IN ROW EXCLUSIVE MODE;

			SELECT INTO orig * FROM  \' || quote_ident(tablename) || \' WHERE \' || quote_ident(keycol)    || \' = NEW.\' || quote_ident(keycol) || \';

			IF NOT FOUND THEN
				RETURN NEW;
			END IF;

			UPDATE \' || quote_ident(tablename) || \' SET \'; 

	arraydims := array_dims(updatecols);
	FOR i IN 1 .. (substring(arraydims from (position(\':\' in arraydims) + 1 ) for ( position(\']\' in arraydims) - (position(\':\' in arraydims) + 1 ) )))::INT LOOP
		trig := trig || quote_ident(updatecols[i]) || \' = COALESCE( NEW.\' || quote_ident(updatecols[i]) || \', orig.\' || quote_ident(updatecols[i]) || \'), \';
	END LOOP;

	trig := substring( trig from 0 for (character_length(trig) - 1));

	trig := trig || \' WHERE \' || quote_ident(keycol)    || \' = NEW.\' || quote_ident(keycol) || \';
			RETURN NULL;
		END;
		\'\' LANGUAGE \'\'plpgsql\'\';
	\';

	EXECUTE trig;
	EXECUTE \'
		CREATE TRIGGER "\' || tablename || \'_merge_on_insert_trig" BEFORE INSERT
			ON \' || quote_ident(tablename) || \' FOR EACH ROW
			EXECUTE PROCEDURE "\' || tablename || \'_merge_on_insert_proc" ();
	\';

	RETURN \'FUNCTION \' || tablename || \'_merge_on_insert_proc (); TRIGGER \' || tablename || \'_merge_on_insert_trig;\';
END;

';

CREATE OR REPLACE FUNCTION remove_merge_on_insert (
	TEXT  -- table name
 ) RETURNS TEXT
   RETURNS NULL ON NULL INPUT
   SECURITY INVOKER
   LANGUAGE 'plpgsql'
   AS '

BEGIN
	EXECUTE \'DROP FUNCTION "\' || $1 || \'_merge_on_insert_proc" () CASCADE;\';
	RETURN \'FUNCTION \' || $1 || \'_merge_on_insert_proc (); TRIGGER \' || $1 || \'_merge_on_insert_trig;\';
END;

';


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