[postgis-users] Postgres not using GiST index in a lateral join

[Igor Stassiy]

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
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