Re: [GENERAL] Changing between ORDER BY DESC and ORDER BY ASC
On Fri, Aug 15, 2008 at 9:35 PM, William Garrison [EMAIL PROTECTED]wrote: Is there an easy way to write one single query that can alternate between ASC and DESC orders? Ex: Take a look at this link http://www.mail-archive.com/pgsql-general@postgresql.org/msg111788.html -- Regards, Sergey Konoplev
Re: [GENERAL] Changing between ORDER BY DESC and ORDER BY ASC
http://www.mail-archive.com/pgsql-general@postgresql.org/msg111788.htmlprobably won't match an index, because ASC or DESC ordering depends NOT on the table's data, but on the function parameter. Unfortunately the planner does not recognize the following case: CREATE TABLE public.prime ( num NUMERIC NOT NULL, CONSTRAINT prime_pkey PRIMARY KEY(num) ) WITH OIDS; CREATE INDEX prime_idx ON public.prime USING btree ((CASE WHEN true THEN num ELSE (- num) END)); CREATE OR REPLACE FUNCTION public.prime_test (a boolean) RETURNS SETOF integer AS $body$ DECLARE rec RECORD; BEGIN FOR rec IN select * from prime order by case when a then num else -num end limit 20 LOOP RETURN NEXT rec.num; END LOOP; END; $body$ LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER; EXPLAIN ANALYZE select * from prime_test(true); -- hundreds of seconds - so the index is not used Seems the planner does not understand that a variable is constant true within the query and does not use prime_idx index (in spite of prime_idx is defined dummyly as CASE WHEN true THEN ... ELSE ... END). William, you may try to use EXECUTE instruction with customly built query with ASC or DESC inserted. On Mon, Aug 18, 2008 at 3:31 PM, Sergey Konoplev [EMAIL PROTECTED] wrote: On Fri, Aug 15, 2008 at 9:35 PM, William Garrison [EMAIL PROTECTED]wrote: Is there an easy way to write one single query that can alternate between ASC and DESC orders? Ex: Take a look at this link http://www.mail-archive.com/pgsql-general@postgresql.org/msg111788.html -- Regards, Sergey Konoplev
Re: [GENERAL] Changing between ORDER BY DESC and ORDER BY ASC
On Aug 15, 2008, at 12:35 PM, William Garrison wrote: Is there an easy way to write one single query that can alternate between ASC and DESC orders? Ex: CREATE OR REPLACE FUNCTION GetSomeStuff(_start integer, _count integer, _sortDesc boolean) RETURNS SETOF text AS $BODY$ SELECT something FROM whatever WHERE whatever ORDER BY another_column OFFSET $1 LIMIT $2 ($4 = true ? 'DESC' : 'ASC'); $BODY$ LANGUAGE 'sql' VOLATILE; I can think of a few ways, but I am hoping for something more elegant. 1) In my case another_column is numeric, so I could multiple by negative one if I want it in the other order. Not sure what this does to the optimizer if the column is indexed or not. In my experience, it's pretty rare for an index to be used to satisfy an ORDER BY. 2) I could write the statement twice, once with ASC and once with DESC, and then use IF/ELSE structure to pick one. 3) I could generate the statement dynamically. I am hoping there is some super secret extension that can handle this. This seems like one of those foolish things in SQL, where it is too declarative. ASC and DESC should be parameters to order by, not a part of the syntax. But I digress... any other suggestions? None that I can think of, unfortunately. It might not be horribly hard to allow plpgsql to use a variable for ASC vs DESC; that might be your best bet. -- Decibel!, aka Jim C. Nasby, Database Architect [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 smime.p7s Description: S/MIME cryptographic signature
[GENERAL] Changing between ORDER BY DESC and ORDER BY ASC
Is there an easy way to write one single query that can alternate between ASC and DESC orders? Ex: CREATE OR REPLACE FUNCTION GetSomeStuff(_start integer, _count integer, _sortDesc boolean) RETURNS SETOF text AS $BODY$ SELECT something FROM whatever WHERE whatever ORDER BY another_column OFFSET $1 LIMIT $2 ($4 = true ? 'DESC' : 'ASC'); $BODY$ LANGUAGE 'sql' VOLATILE; I can think of a few ways, but I am hoping for something more elegant. 1) In my case another_column is numeric, so I could multiple by negative one if I want it in the other order. Not sure what this does to the optimizer if the column is indexed or not. 2) I could write the statement twice, once with ASC and once with DESC, and then use IF/ELSE structure to pick one. 3) I could generate the statement dynamically. I am hoping there is some super secret extension that can handle this. This seems like one of those foolish things in SQL, where it is too declarative. ASC and DESC should be parameters to order by, not a part of the syntax. But I digress... any other suggestions? -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
