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

[Paul Ramsey]

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 at gmail.com> 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