difference in query performance due to the inclusion of a polygon geometry field

Dear users,
we are facing with a strange efficiency problem.

We have two tables:
1) the first largest (30000 rows) contains a timestamp field (named
"tempo_calcolo") and other fields.
2) the second contains only few rows (150), a PostGIS geometric POLYGON
field (named "the_geom") and other fields.

The query we want to execute is a join between the two table based on a
common field, filtered by the timestamp field and ordered by the same
field.

Including and excluding the PostGIS geometric POLYGON field (that is not
used neither in the joining or in the filtering) the execution times are
really different.

Without the geometric field it takes around 86 ms !!
With the geometric field it takes around 14000 ms !!

These are the EXPLAIN ANALYSE results obtained when we perform the query
without the geometric field:
____________________________
"Sort (cost=1217.29..1240.40 rows=9246 width=133) (actual
time=2.964..3.095 rows=1474 loops=1)"
" Sort Key: storico_zone_allertamento.tempo_calcolo"
" Sort Method: quicksort Memory: 440kB"
" InitPlan 1 (returns $0)"
" -> Result (cost=0.00..0.02 rows=1 width=0) (actual
time=0.022..0.022 rows=1 loops=1)"
" -> Hash Join (cost=6.01..608.21 rows=9246 width=133) (actual
time=0.143..1.916 rows=1474 loops=1)"
" Hash Cond: (storico_zone_allertamento.gid_fk =
zone_allertamento.gid)"
" -> Index Scan using
index_storico_zone_allertamento_tempo_calcolo on
storico_zone_allertamento (cost=0.00..475.06 rows=9246 width=112)
(actual time=0.042..0.440 rows=1474 loops=1)"
" Index Cond: (tempo_calcolo > $0)"
" -> Hash (cost=4.34..4.34 rows=134 width=25) (actual
time=0.090..0.090 rows=134 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 8kB"
" -> Seq Scan on zone_allertamento (cost=0.00..4.34
rows=134 width=25) (actual time=0.004..0.035 rows=134 loops=1)"
"Total runtime: 3.285 ms"
__________________________

These are the EXPLAIN ANALYSE results obtained when we perform the query
with the geometric field:
___________________________
"Sort (cost=776019.29..776042.40 rows=9246 width=196111) (actual
time=3.039..3.163 rows=1474 loops=1)"
" Sort Key: storico_zone_allertamento.tempo_calcolo"
" Sort Method: quicksort Memory: 567kB"
" InitPlan 1 (returns $0)"
" -> Result (cost=0.00..0.02 rows=1 width=0) (actual
time=0.009..0.009 rows=1 loops=1)"
" -> Hash Join (cost=6.01..608.21 rows=9246 width=196111) (actual
time=0.139..1.957 rows=1474 loops=1)"
" Hash Cond: (storico_zone_allertamento.gid_fk =
zone_allertamento.gid)"
" -> Index Scan using
index_storico_zone_allertamento_tempo_calcolo on
storico_zone_allertamento (cost=0.00..475.06 rows=9246 width=112)
(actual time=0.029..0.431 rows=1474 loops=1)"
" Index Cond: (tempo_calcolo > $0)"
" -> Hash (cost=4.34..4.34 rows=134 width=196003) (actual
time=0.091..0.091 rows=134 loops=1)"
" Buckets: 1024 Batches: 1 Memory Usage: 19kB"
" -> Seq Scan on zone_allertamento (cost=0.00..4.34
rows=134 width=196003) (actual time=0.006..0.038 rows=134 loops=1)"
"Total runtime: 3.355 ms"
__________________________

It seems that the difference is primary in the "width" and then in the
"cost" of the Sort.

A similar task performed on two similar tables (both larger than the
previous ones!!) but including and excluding a PostGIS geometric POINT
(!!) field instead of POLYGON one, doesn't result in significant query
execution time difference.

Do you have please any suggestion to improve our query efficiency??

Thanks in advance

Ivan and Mauro

--
Ti prego di cercare di non inviarmi files .dwg, .doc, .xls, .ppt.
Preferisco formati liberi.
Please try to avoid to send me .dwg, .doc, .xls, .ppt files.
I prefer free formats.
http://it.wikipedia.org/wiki/Formato_aperto
http://en.wikipedia.org/wiki/Open_format

Ivan Marchesini
Perugia (Italy)
Socio fondatore GFOSS "Geospatial Free and Open Source Software" http://www.gfoss.it
e-mail: ivan(dot)marchesini(at)irpi(dot)cnr(dot)it
ivan(dot)marchesini(at)gmail(dot)com
fax (mailfax): +39 1782092534
jabber: geoivan73(at)jabber(dot)org
skype: geoivan73