From: Szabo Zoltan <col@mportal.hu>
Subject: index problem
Date: Mon, 15 Oct 2001 21:41:10 +0200
Organization: Hub.Org Networking Services (http://www.hub.org)
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Message-ID: <3BCB3BD6.70500@mportal.hu>
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To: pgsql-sql@postgresql.org

Hi,

I have that:

1)
db=> explain select pxygy_pid from prog_dgy_xy where pxygy_pid=12121;
NOTICE:  QUERY PLAN:

Group  (cost=0.00..29970.34 rows=921 width=4)
   ->  Index Scan using progdgyxy_idx2 on prog_dgy_xy 
(cost=0.00..29947.32 rows=9210 width=4)

than:
2)
db=> explain select pxygy_pid from prog_dgy_xy where pxygy_pid>12121;
NOTICE:  QUERY PLAN:

Group  (cost=66927.88..67695.39 rows=30700 width=4)
   ->  Sort  (cost=66927.88..66927.88 rows=307004 width=4)
         ->  Seq Scan on prog_dgy_xy  (cost=0.00..32447.66 rows=307004 
width=4)

I making some banchmarks on: oracle vs postgres vs mysql. And this is 
breaking me now;) Mysql and oracle width same table and index use that 
index on pxygy_pid;
I had vacuum before.

Time with mysql:

bash-2.04$ time echo " select count(*) from PROG_DGY_XY where 
pxygy_pid>12121;" | mysql -uuser -ppasswd db
count(*)
484984

real    0m13.761s
user    0m0.008s
sys     0m0.019s

Time with postgres:
bash-2.04$ time echo "select count(*) from PROG_DGY_XY where 
pxygy_pid>12121 " | psql -Uuser db
  count
--------
  484984
(1 row)


real    0m22.480s
user    0m0.011s
sys     0m0.021s

And this is just a little part of another selects joining tables, but 
because this index is not used, selecting from 2 tables (which has 
indexes, and keys on joining collumns) takes extrem time for postgres: 
2m14.978s while for mysql it takes: 0m0.578s !!!

this select is: select distinct 
PROG_ID,PROG_FTYPE,PROG_FCASTHOUR,PROG_DATE from PROG_DATA, PROG_DGY_XY 
  where prog_id=pxygy_pid  order by prog_date,prog_ftype,prog_fcasthour

indexes:
PROG_DATA:
create index prod_data_idx1 on prog_data 
(prog_date,prog_ftype,prog_fcasthour);
prog_id is primary key

PROG_DGY_XY:
create unique index progdgyxy_idx1 on PROG_DGY_XY 
(PXYGY_PID,PXYGY_X,PXYGY_Y);
create index progdgyxy_idx2 on PROG_DGY_XY (PXYGY_PID);


Thx
CoL