Trying to understand why query planer changes the plan from effective
one to ineffective one when I change the offset in the LIMIT. Also,
thankfully accepting RTFM pointers to the actual FMs.
Setup is: 3 tables with 0.5M to 1.5M records
While tuning indexes for the following query
SELECT c.id, c.name, c.owner
FROM catalog c, catalog_securitygroup cs, catalog_university cu
WHERE c.root < 50
AND cs.catalog = c.id
AND cu.catalog = c.id
AND cs.securitygroup < 200
AND cu.university < 200
ORDER BY c.name
LIMIT 50 OFFSET 100
I managed to bring it to ~3ms with the following plan
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------
Limit (cost=15141.07..22711.60 rows=50 width=59)
-> Nested Loop (cost=0.00..30052749.38 rows=198485 width=59)
-> Nested Loop (cost=0.00..705519.23 rows=147500 width=63)
-> Index Scan using test2 on catalog c
(cost=0.00..241088.93 rows=147500 width=59)
Index Cond: (root < 50)
-> Index Scan using catalog_university_pkey on
catalog_university cu (cost=0.00..3.14 rows=1 width=4)
Index Cond: ((cu.catalog = c.id) AND
(cu.university < 200))
-> Index Scan using catalog_securitygroup_pkey on
catalog_securitygroup cs (cost=0.00..196.48 rows=199 width=4)
Index Cond: ((cs.catalog = c.id) AND (cs.securitygroup
< 200))
But when I change the OFFSET in the limit to 500 it goes to ~500ms
with following plan
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=61421.34..61421.46 rows=50 width=59)
-> Sort (cost=61420.09..61916.30 rows=198485 width=59)
Sort Key: c.name
-> Merge Join (cost=45637.87..51393.33 rows=198485
width=59)
Merge Cond: (c.id = cs.catalog)
-> Merge Join (cost=48.95..440699.65 rows=147500
width=63)
Merge Cond: (c.id = cu.catalog)
-> Index Scan using catalog_pkey on catalog c
(cost=0.00..78947.35 rows=147500 width=59)
Filter: (root < 50)
-> Index Scan using catalog_university_pkey on
catalog_university cu (cost=0.00..358658.68 rows=499950 width=4)
Index Cond: (cu.university < 200)
-> Materialize (cost=45527.12..48008.19 rows=198485
width=4)
-> Sort (cost=45527.12..46023.34 rows=198485
width=4)
Sort Key: cs.catalog
-> Seq Scan on catalog_securitygroup cs
(cost=0.00..25345.76 rows=198485 width=4)
Filter: (securitygroup < 200)
Thanks for your time
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