Обсуждение: Why are these queries so different in time?
Hi, I have a question about performance querying a 7.4 database. The orginal generated query was SELECT DISTINCT _compound0.object_id AS "ObjectId" FROM amaze._compound _compound0 LEFT JOIN amaze._product _product7ON (_compound0.object_id = _product7.compound) LEFT JOIN amaze._database_object _database_object11 ON (_product7.reaction = _database_object11.object_id) LEFT JOIN amaze._educt _educt2 ON (_compound0.object_id = _educt2.compound) LEFT JOIN amaze._database_object _database_object6 ON (_educt2.reaction = _database_object6.object_id) WHERE ( _database_object6.label = '2-Isopropylmalate + CoA <= Acetyl-CoA + 3-Methyl-2-oxobutanoate + H2O' OR _database_object11.label = '2-Isopropylmalate + CoA <= Acetyl-CoA + 3-Methyl-2-oxobutanoate + H2O' ); This on take a huge time to perform, which may come to a timeout on the front-end application that uses the database. So, I decided to modify manually the query like this: SELECT DISTINCT _compound0.object_id AS "ObjectId" FROM amaze._compound _compound0 LEFT JOIN amaze._product _product7ON (_compound0.object_id = _product7.compound) LEFT JOIN amaze._database_object _database_object11 ON (_product7.reaction = _database_object11.object_id) WHERE ( _database_object11.label = '2-Isopropylmalate + CoA <= Acetyl-CoA + 3-Methyl-2-oxobutanoate + H2O' ) UNION SELECT DISTINCT _compound0.object_id AS "ObjectId" FROM amaze._compound _compound0 LEFT JOIN amaze._educt _educt2ON (_compound0.object_id = _educt2.compound) LEFT JOIN amaze._database_object _database_object6 ON (_educt2.reaction = _database_object6.object_id) WHERE ( _database_object6.label = '2-Isopropylmalate + CoA <= Acetyl-CoA + 3-Methyl-2-oxobutanoate + H2O' ) This should give the same result set, but it's really faster than the previous one, more than one thousand time faster. Is there a reason for this huge difference of performance? Thanks in advance. -- Olivier Hubaut North Bears Team SCMBB - ULB
Olivier Hubaut wrote: > Hi, I have a question about performance querying a 7.4 database. The > orginal generated query was > > SELECT DISTINCT _compound0.object_id AS "ObjectId" > FROM (4 LEFT JOINS then a couple of WHERE conditions on 2 tables) > This on take a huge time to perform, which may come to a timeout on the > front-end application that uses the database. > So, I decided to modify manually the query like this: > > SELECT DISTINCT _compound0.object_id AS "ObjectId" > FROM (Two lots of 2 x Left-joins, unioned together) > This should give the same result set, but it's really faster than the > previous one, more than one thousand time faster. > Is there a reason for this huge difference of performance? You're probably processing 1000 more rows in the first example. It's probably running the LEFT JOIN across all the tables then restricting the results in the WHERE. As it happens you're throwing away duplicates with DISTINCT and/or UNION anyway, so you never get to see the results. Try an EXPLAIN ANALYSE of the first example and see if the rows= parts indicate very large numbers of rows being processed. To make it faster I'd remove the LEFT JOINs, since your WHERE conditions seem to rule out the NULL cases anyway. -- Richard Huxton Archonet Ltd