RE : Increase performance of a UNION query that thakes 655.07 msec to be runned ?

re-Hello,

As suggested by Tom, I've removed the distinct and tried it's query :

levure=> explain analyze select initiale from (
levure(> select lower(substr(l_name,1,1)) as initiale from people
levure(> union all
levure(> select lower(substr(org_name,1,1)) as initiale from
organizations
levure(> ) ss
levure-> group by initiale order by initiale;
QUERY
PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
-
Group (cost=1018.48..1074.32 rows=1117 width=17) (actual
time=783.47..867.61 rows=39 loops=1)
-> Sort (cost=1018.48..1046.40 rows=11167 width=17) (actual
time=782.18..801.68 rows=11167 loops=1)
Sort Key: initiale
-> Subquery Scan ss (cost=0.00..267.67 rows=11167 width=17)
(actual time=0.23..330.31 rows=11167 loops=1)
-> Append (cost=0.00..267.67 rows=11167 width=17)
(actual time=0.22..263.69 rows=11167 loops=1)
-> Subquery Scan "*SELECT* 1" (cost=0.00..87.93
rows=4093 width=15) (actual time=0.22..79.51 rows=4093 loops=1)
-> Seq Scan on people (cost=0.00..87.93
rows=4093 width=15) (actual time=0.20..53.82 rows=4093 loops=1)
-> Subquery Scan "*SELECT* 2" (cost=0.00..179.74
rows=7074 width=17) (actual time=0.24..146.12 rows=7074 loops=1)
-> Seq Scan on organizations
(cost=0.00..179.74 rows=7074 width=17) (actual time=0.23..100.70
rows=7074 loops=1)
Total runtime: 874.79 msec
(10 rows)

That seems to be 200 msec longer that my first query... Indeed, I've
noticed something strange : now, if I rerun my initial query, I get
worse runtime than this morning :

levure=> EXPLAIN ANALYZE SELECT lower(substr(l_name, 1, 1)) AS initiale
FROM people
levure-> UNION
levure-> SELECT lower(substr(org_name, 1, 1)) AS initiale FROM
organizations
levure-> ORDER BY initiale;
QUERY
PLAN
------------------------------------------------------------------------
------------------------------------------------------------------------
Sort (cost=1130.85..1133.64 rows=1117 width=17) (actual
time=802.52..802.58 rows=39 loops=1)
Sort Key: initiale
-> Unique (cost=1018.48..1074.32 rows=1117 width=17) (actual
time=712.04..801.83 rows=39 loops=1)
-> Sort (cost=1018.48..1046.40 rows=11167 width=17) (actual
time=712.03..732.63 rows=11167 loops=1)
Sort Key: initiale
-> Append (cost=0.00..267.67 rows=11167 width=17)
(actual time=0.21..263.54 rows=11167 loops=1)
-> Subquery Scan "*SELECT* 1" (cost=0.00..87.93
rows=4093 width=15) (actual time=0.20..80.47 rows=4093 loops=1)
-> Seq Scan on people (cost=0.00..87.93
rows=4093 width=15) (actual time=0.19..54.14 rows=4093 loops=1)
-> Subquery Scan "*SELECT* 2" (cost=0.00..179.74
rows=7074 width=17) (actual time=0.28..144.82 rows=7074 loops=1)
-> Seq Scan on organizations
(cost=0.00..179.74 rows=7074 width=17) (actual time=0.27..99.06
rows=7074 loops=1)
Total runtime: 806.47 msec
(11 rows)

I don't understand why this runtime has changed because no data has been
added/updated/deleted since several weeks (I'm working on a copy of the
production database. And this copy is not accessible for the users).

My PostgreSQL version is PostgreSQL 7.3.2, I have to ask to the
administrator if it can be upgraded to 7.4 in the production server.

Thanks in advance for your help.

---------------------------------------
Bruno BAGUETTE - pgsql-ml(at)baguette(dot)net