Slow query in postgresql geonames db despite indexes - postgresql

Question

I imported all tables from http://www.geonames.org/ into my local postgresql 9.5.3.0 database and peppered it with indexes like so:
create extension pg_trgm;
CREATE INDEX name_trgm_idx ON geoname USING GIN (name gin_trgm_ops);
CREATE INDEX fcode_trgm_idx ON geoname USING GIN (fcode gin_trgm_ops);
CREATE INDEX fclass_trgm_idx ON geoname USING GIN (fclass gin_trgm_ops);
CREATE INDEX alternatename_trgm_idx ON alternatename USING GIN (alternatename gin_trgm_ops);
CREATE INDEX isolanguage_trgm_idx ON alternatename USING GIN (isolanguage gin_trgm_ops);
CREATE INDEX alt_geoname_id_idx ON alternatename (geonameid)
And now I would like to query the country names in different languages and cross reference the geonames attributes with these alternative names like so:
select g.geonameid as geonameid ,a.alternatename as name,g.country as country, g.fcode as fcode
from geoname g,alternatename a
where
a.isolanguage=LOWER('de')
and a.alternatename ilike '%Sa%'
and (a.ishistoric = FALSE OR a.ishistoric IS NULL)
and (a.isshortname = TRUE OR a.isshortname IS NULL)
and a.geonameid = g.geonameid
and g.fclass='A'
and g.fcode ='PCLI';
Unfortunately though this query takes as long as 13 to 15 seconds on an octacore machine with a fast SSD. 'Explain analyze verbose' shows this:
Nested Loop (cost=0.43..237138.04 rows=1 width=25) (actual time=1408.443..10878.115 rows=15 loops=1)
Output: g.geonameid, a.alternatename, g.country, g.fcode
-> Seq Scan on public.alternatename a (cost=0.00..233077.17 rows=481 width=18) (actual time=0.750..10862.089 rows=2179 loops=1)
Output: a.alternatenameid, a.geonameid, a.isolanguage, a.alternatename, a.ispreferredname, a.isshortname, a.iscolloquial, a.ishistoric
Filter: (((a.alternatename)::text ~~* '%Sa%'::text) AND ((a.isolanguage)::text = 'de'::text))
Rows Removed by Filter: 10675099
-> Index Scan using pk_geonameid on public.geoname g (cost=0.43..8.43 rows=1 width=11) (actual time=0.006..0.006 rows=0 loops=2179)
Output: g.geonameid, g.name, g.asciiname, g.alternatenames, g.latitude, g.longitude, g.fclass, g.fcode, g.country, g.cc2, g.admin1, g.admin2, g.admin3, g.admin4, g.population, g.elevation, g.gtopo30, g.timezone, g.moddate
Index Cond: (g.geonameid = a.geonameid)
Filter: ((g.fclass = 'A'::bpchar) AND ((g.fcode)::text = 'PCLI'::text))
Rows Removed by Filter: 1
Which to me seems to indicate that somehow a sequence scan is performed on 481 rows (which I deem to be fairly low), but nevertheless takes very long. I currently can't make sense of this. Any ideas?

Answer 1

The trigrams only work if you have minimum of 3 characters you're searching for %Sa% won't work, %foo% will. However your indexes are still not good enough. Depending on what parameters are dynamic use multicolumn or filtered indexes:
CREATE INDEX jkb1 ON geoname(fclass, fcode, geonameid, country);
CREATE INDEX jkb2 ON geoname(geonameid, country) WHERE fclass = 'A' AND fcode = 'PCLI';
Same for the other table:
CREATE INDEX jkb3 ON alternatename(geonameid, alternatename) WHERE (a.ishistoric = FALSE OR a.ishistoric IS NULL)
AND (a.isshortname = TRUE OR a.isshortname IS NULL) AND isolanguage=LOWER('de')