Re: Slow sub-selects, max and count(*)

Hi,

I don't think thatyou gave enough information to get any direct help, for
example, what are these "sub-selects"?

I often see performance problems arising from procedural code that puts
selects that don't use indexes inside loops. That's a plain programming
issue, and understanding your system.

Another situation I encountered recently was using "in" sub selects.
Recoding them as "exists" or "not exists" as the case may be turned a query
that ran for 18hours with no sign of ending (before I killed it) into a
query that ran in a couple of minutes.

I always try to avoid procedural code in such batch updates. If it can be
encoded into one insert then it is usually better, It just depends on what
you are doing.

----- Original Message -----
From: Richard Sydney-Smith
To: pgsql-sql(at)postgresql(dot)org
Sent: Thursday, February 05, 2004 10:48 AM
Subject: [SQL] Slow sub-selects, max and count(*)

I have a procedure ( included below with table definition) to import end
of day quotes into a table fsechist.

The issue is with the speed ( or lackof it) that the procedure proceeds.
Apparently it is the subselects that are the worst issue and I have tried to
replace these. Also max() and count() refuse to use indexes. I have replaced
max() with (select ... as mx from ... order by ... desc limit 1) and this is
much faster but appear to have to avoid count() as I can not find a way to
point this at an index.

Please, I am sure that there is a better way to do this. And 5 to 7
minutes to insert a day is really too slow. Each days import table contains
about 3200 records. Total table size for fsechist is about 2.5 million
records.

If you could point me in the right direction here it would be much
appreciated.

thanks

Richard Sydney-Smith

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

CREATE OR REPLACE FUNCTION public.import_sharedata(varchar)
RETURNS bool AS
'

declare
filnam alias for $1;
alldone bool := true;
/* cnt integer := 0; */ /* count of number inserted not used */

begin
/*
Imports a text file containing end of day price quotes
filnam should be in double backslash format e.g.
c:\\data\\sharequotes\\quotes.dat

Ensures that it only brings in quotes for companies we have listed in
our database
and ensures only the latest copy of the quote is kept for each day

Import table has 7 columns in CSV tab delimited ascii format.
EzyChart format has prices in cents,
METASTOCK in dollars
MetaStock +100 has the volume divided by 100.

Depending on your import data source you may need to process the
temp_shareprices so that the
values are in Dollars , not cents and the volume is per unit not per
100 units.

Present calculation is for EzyChart format.

Both tables have to have a 4 digit year yyyymmdd

Warning : If you have the wrong format this procedure will import
garbage into your database !!!

*/

perform drop_table(\'temp_shareprices\');

create table temp_shareprices(
ticker char(10) null,
dte char(8),
v1 numeric(19,4) not null default 0,
v2 numeric(19,4) not null default 0,
v3 numeric(19,4) not null default 0,
v4 numeric(19,4) not null default 0,
v5 numeric(19,4) not null default 0
);
execute \'copy temp_shareprices from \'||quote_literal(filnam)||\'
delimiter \'||quote_literal(\',\');

-- extend yr to 4 digits, alter quotes to dollars

update temp_shareprices set dte =
\'20\'||dte,v1=v1/100,v2=v2/100,v3=v3/100,v4=v4/100;

-- add in any new company tickers
insert into fsecurity (sec_cde) select ticker from temp_shareprices where
length(trim(ticker))=3 and ticker not in ( select sec_cde from fsecurity);

-- ====================
-- remove rows from temp_shareprices where we do not have a corresponding
security

-- using sub-select is slow
-- delete from temp_shareprices where ticker not in (select sec_cde from
fsecurity);

-- can just delete ones where length<>3 as insert above ensures all length
3 are inserted. All ASX codes are 3char long
delete from temp_shareprices where length(trim(ticker))<>3;

-- Attempt to remove without limiting to length and not using sub select
does not work
--update temp_shareprices set ticker = s.sec_cde from temp_shareprices t
left outer join fsecurity s on t.ticker = s.sec_cde;
--delete from temp_shareprices where ticker is null;
-- ====================

-- put indexes on temp table
CREATE INDEX temphist_tick ON temp_shareprices USING btree (ticker);
CREATE INDEX temphist_dte ON temp_shareprices USING btree (dte);

-- ====================
-- Scan the input table and delete as appropriate from fsechist

-- Using sub select to remove any pre-existing records in fsechist very
slow
-- delete from fsechist where hist_q in (select hist_q from fsechist,
temp_shareprices where hist_tick = ticker and dte = hist_date);

-- Tried to replace subselect
update fsechist set hist_tick = \'@@\' from fsechist h, temp_shareprices t
where t.ticker = h.hist_tick and h.hist_date = t.dte;
delete from fsechist where hist_tick = \'@@\';
-- ====================

-- insert newvalues
insert into
fsechist(hist_tick,hist_date,hist_v1,hist_v2,hist_v3,hist_v4,hist_vol)
select ticker,dte,v1,v2,v3,v4,v5 from temp_shareprices;

-- count the number inserted
-- select count(*) into cnt from temp_shareprices;

-- perform drop_table(\'temp_shareprices\');

return alldone;

end;'
LANGUAGE 'plpgsql' VOLATILE;

Note : drop_table is a simple procedure that drops a procedure by name iff
it exists.

==============================================
Definition of Fsechist

CREATE TABLE public.fsechist
(
hist_q int4 NOT NULL DEFAULT nextval('hist_q'::text),
hist_v1 numeric(19,4),
hist_v2 numeric(19,4),
hist_v3 numeric(19,4),
hist_v4 numeric(19,4),
hist_vol numeric(19,4),
hist_deltar numeric(19,4),
hist_deltag numeric(19,4),
hist_date char(8),
hist_tick char(10),
hist_announce char(1),
CONSTRAINT fsechist_pkey PRIMARY KEY (hist_q)
) WITH OIDS;

CREATE INDEX fsechist_date ON public.fsechist USING btree (hist_date);

CREATE UNIQUE INDEX fsechist_q ON public.fsechist USING btree (hist_q);

CREATE INDEX fsechist_tick ON public.fsechist USING btree (hist_tick);