Stop writing so many subqueries, think in joins; the poor planner! SELECT DISTINCT ON (a.id) a.id AS a_id, b.id AS b_id FROM a JOIN b ON ST_Contains(b.shape, a.shape) WHERE b.kind != 1 Also, the DISTINCT ON syntax (PgSQL custom) lets you winnow a result set down to just one of the inputs. P. On Wed, Mar 4, 2015 at 6:36 AM, Igor Stassiy <istassiy@xxxxxxxxx> wrote: > Hello, > > I have a query plan optimization question. It is formatted nicely on > > http://stackoverflow.com/questions/28856452/postgres-not-using-gist-index-in-lateral-join > > But here is a copy for the archive: > > Here is the setup: > > CREATE EXTENSION postgis; > DROP TABLE IF EXISTS A; > DROP TABLE IF EXISTS B; > CREATE TABLE A(shape Geometry, id INT); > CREATE TABLE B(shape Geometry, id INT, kind INT); > CREATE INDEX ON A USING GIST (shape); > CREATE INDEX ON B USING GIST (shape); > > I am running the following commands: > > ANALYZE A; > ANALYZE B; > > -- for each row in A, select exactly one row in B (if there is one) > -- such that B contains geometry of A > EXPLAIN SELECT * FROM (SELECT A.id as aid, (SELECT B.id FROM B WHERE > ST_Contains(B.shape, A.shape) AND B.kind != 1 LIMIT 1) as bid FROM A) AS > TMP; > > which gives me { "Plan": { "Node Type": "Seq Scan", "Relation Name": "A", > "Startup Cost": 0.00, "Total Cost": 2606592.33, "Plan Rows": 549745, "Plan > Width": 1646, "Plans": [ { "Node Type": "Limit", "Parent Relationship": > "SubPlan", "Subplan Name": "SubPlan 1", "Startup Cost": 0.00, "Total Cost": > 4.68, "Plan Rows": 1, "Plan Width": 8, "Plans": [ { "Node Type": "Seq Scan", > "Parent Relationship": "Outer", "Relation Name": "B", "Startup Cost": 0.00, > "Total Cost": 4.68, "Plan Rows": 1, "Plan Width": 8, "Filter": "((shape && > A.shape) AND _st_contains(shape, A.shape))" } ] } ] } } > > > Note that there is a sequential scan inside the lateral join, however there > is clearly an index available. However after setting > > set enable_seqscan=false; > > the index is being used. This actually affects runtime significantly (around > 3 times faster) and seems that postgres should figure things like that > automatically. { "Plan": { "Node Type": "Seq Scan", "Relation Name": "A", > "Startup Cost": 10000000000.00, "Total Cost": 10004716493.85, "Plan Rows": > 549450, "Plan Width": 1677, "Plans": [ { "Node Type": "Limit", "Parent > Relationship": "SubPlan", "Subplan Name": "SubPlan 1", "Startup Cost": 0.00, > "Total Cost": 8.52, "Plan Rows": 1, "Plan Width": 8, "Plans": [ { "Node > Type": "Index Scan", "Parent Relationship": "Outer", "Scan Direction": > "NoMovement", "Index Name": "B_shape_idx", "Relation Name": "B", "Startup > Cost": 0.00, "Total Cost": 8.52, "Plan Rows": 1, "Plan Width": 8, "Index > Cond": "(shape && A.shape)", "Filter": "_st_contains(shape, A.shape)" } ] } > ] } } > > Is there any way to tell postgres to use index in a less hacky way? Possibly > by rewriting the query? From what I understand the use of set enable_... is > not recommended in production. > > When you actually run the commands above it will give > > { "Plan": { "Node Type": "Seq Scan", "Relation Name": "a", "Alias": "a", > "Startup Cost": 0.00, "Total Cost": 10372.75, "Plan Rows": 1230, "Plan > Width": 36, "Plans": [ { "Node Type": "Limit", "Parent Relationship": > "SubPlan", "Subplan Name": "SubPlan 1", "Startup Cost": 0.14, "Total Cost": > 8.41, "Plan Rows": 1, "Plan Width": 4, "Plans": [ { "Node Type": "Index > Scan", "Parent Relationship": "Outer", "Scan Direction": "NoMovement", > "Index Name": "b_shape_idx", "Relation Name": "b", "Alias": "b", "Startup > Cost": 0.14, "Total Cost": 8.41, "Plan Rows": 1, "Plan Width": 4, "Index > Cond": "(shape && a.shape)", "Filter": "((kind <> 1) AND _st_contains(shape, > a.shape))" } ] } ] } } > > Unfortunately I cannot provide data to reproduce the query plan results. > > Thanks, > Igor -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
