On Monday 09 April 2007 05:09:53 s d wrote: > Hi, > I am trying to figure out how to debug a performance problem / use psql > explain. The table in question is: > # \d word_association; > Table "public.word_association" > Column | Type | Modifiers > --------+------------------------+-------------------- > word1 | character varying(128) | not null > word2 | character varying(128) | not null > count | integer | not null default 0 > Indexes: > "word1_word2_comb_unique" unique, btree (word1, word2) > "word1_hash_index" hash (word1) > "word2_hash_index" hash (word2) > "word_association_count_index" btree (count) > "word_association_index1_1" btree (word1) > "word_association_index2_1" btree (word2) > > It has multiple indices since i wanted to see which one the planner choses. > > > # explain select * FROM word_association WHERE (word1 = 'bdss' OR > word2 = 'bdss') AND count >= 10; > QUERY PLAN > --------------------------------------------------------------------------- >--------------------- Bitmap Heap Scan on word_association > (cost=11.53..1192.09 rows=155 width=22) Recheck Cond: (((word1)::text = > 'bdss'::text) OR ((word2)::text = 'bdss'::text)) > Filter: (count >= 10) > -> BitmapOr (cost=11.53..11.53 rows=364 width=0) > -> Bitmap Index Scan on word_association_index1_1 > (cost=0.00..5.79 rows=190 width=0) > Index Cond: ((word1)::text = 'bdss'::text) > -> Bitmap Index Scan on word_association_index2_1 > (cost=0.00..5.67 rows=174 width=0) > Index Cond: ((word2)::text = 'bdss'::text) > (8 rows) > > The questions: > 1. i can undestand where the cost=11.53 came from but where did the > 1192.09 come form? The values are in milli right ? > 2. the query takes in reality much longer than 1 second. > > In short, it feels like something is very wrong here (i tried vacuum > analyze and it didn't do much diff). > any ideas ? You need an index on (word1, word2, count). In your current setup it will have to scan all rows that satisfy word1 and word2 to see if count >= 10. jan -- -------------------------------------------------------------- Jan de Visser jdevisser@xxxxxxxxxxxxxxxxxx Baruk Khazad! Khazad ai-menu! --------------------------------------------------------------
