query plan, index scan cost

hi list,

i have a problem with time consuming query. first of all my table structure:

CREATE TABLE nw_tla_2008_4_deu
(
"ID" bigint NOT NULL,
"NET2CLASS" smallint,
"FOW" smallint,
CONSTRAINT nw_tla_2008_4_deu_pkey PRIMARY KEY ("ID"),
)
WITHOUT OIDS;

CREATE INDEX nw_tla_2008_4_deu_fow_idx
ON nw_tla_2008_4_deu
USING btree
("FOW");

CREATE INDEX nw_tla_2008_4_deu_net2class_idx
ON nw_tla_2008_4_deu
USING btree
("NET2CLASS");

CREATE INDEX nw_tla_2008_4_deu_the_geom_gist
ON nw_tla_2008_4_deu
USING gist
(the_geom gist_geometry_ops);
ALTER TABLE nw_tla_2008_4_deu CLUSTER ON nw_tla_2008_4_deu_the_geom_gist;

when i run the following query with explain analyze i get the following result:

EXPLAIN
ANALYZE

SELECT
nw."ID" AS id

FROM
nw_tla_2008_4_deu AS nw

WHERE
expand(st_pointfromtext('POINT(13.7328934 51.049476)',4326), 0.24769615911118054) && nw.the_geom
AND nw."FOW" IN (1,2,3,4,10,17)
AND nw."NET2CLASS" IN (0,1,2,3)

Bitmap Heap Scan on nw_tla_2008_4_deu nw (cost=35375.52..77994.15 rows=11196 width=8) (actual time=13307.830..13368.969 rows=15425 loops=1)

Recheck Cond: ("NET2CLASS" = ANY ('{0,1,2,3}'::integer[]))

Filter: (('0103000020E61000000100000005000000000000C06BF82A40000000A0A0664940000000C06BF82A40000000C009A64940000000E00FF62B40000000C009A64940000000E00FF62B40000000A0A0664940000000C06BF82A40000000A0A0664940'::geometry && the_geom) AND ("FOW" = ANY ('{1,2,3,4,10,17}'::integer[])))

-> BitmapAnd (cost=35375.52..35375.52 rows=12614 width=0) (actual time=13307.710..13307.710 rows=0 loops=1)

-> Bitmap Index Scan on nw_tla_2008_4_deu_the_geom_gist (cost=0.00..1759.12 rows=55052 width=0) (actual time=22.452..22.452 rows=52840 loops=1)

Index Cond: ('0103000020E61000000100000005000000000000C06BF82A40000000A0A0664940000000C06BF82A40000000C009A64940000000E00FF62B40000000C009A64940000000E00FF62B40000000A0A0664940000000C06BF82A40000000A0A0664940'::geometry && the_geom)

-> Bitmap Index Scan on nw_tla_2008_4_deu_net2class_idx (cost=0.00..33610.55 rows=1864620 width=0) (actual time=13284.121..13284.121 rows=2021814 loops=1)

Index Cond: ("NET2CLASS" = ANY ('{0,1,2,3}'::integer[]))

Total runtime: *13.332* ms

running the next query which is only slightly different and has one instead of two and conditions leads to the following result

EXPLAIN
ANALYZE

SELECT
nw."ID" AS id

FROM
nw_tla_2008_4_deu AS nw

WHERE
expand(st_pointfromtext('POINT(13.7328934 51.049476)',4326), 0.24769615911118054) && nw.the_geom
AND nw."FOW" IN (1,2,3,4,10,17)

Bitmap Heap Scan on nw_tla_2008_4_deu nw (cost=1771.34..146161.54 rows=48864 width=8) (actual time=23.285..99.493 rows=47723 loops=1)

Filter: (('0103000020E61000000100000005000000000000C06BF82A40000000A0A0664940000000C06BF82A40000000C009A64940000000E00FF62B40000000C009A64940000000E00FF62B40000000A0A0664940000000C06BF82A40000000A0A0664940'::geometry && the_geom) AND ("FOW" = ANY ('{1,2,3,4,10,17}'::integer[])))

-> Bitmap Index Scan on nw_tla_2008_4_deu_the_geom_gist (cost=0.00..1759.12 rows=55052 width=0) (actual time=22.491..22.491 rows=52840 loops=1)

Index Cond: ('0103000020E61000000100000005000000000000C06BF82A40000000A0A0664940000000C06BF82A40000000C009A64940000000E00FF62B40000000C009A64940000000E00FF62B40000000A0A0664940000000C06BF82A40000000A0A0664940'::geometry && the_geom)

Total runtime: *109*ms

so in both querys there are and conditions. there are two and conditions in the first query and one and condition in the second query. unfortunately i am not an expert in reading the postgre query plan. basically i am wondering why in the first query a second index scan is done whereas in the second query the second index scan is not done. the second query runs hundred times faster then first one which surprising to me.

any ideas?

regards, stefan

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