This I don't get. Why is an index scan not used? Isn't an index supposed
to help when using > < >= <= too?
It should !
Explain Analyze Select count(smiles) from structure where _c >= 30
Aggregate (cost=196033.74..196033.74 rows=1 width=32) (actual time=42133.432..42133.434 rows=1
loops=1)
-> Seq Scan on structure (cost=0.00..191619.56 rows=1765669 width=32) (actual
time=8050.437..42117.062 rows=1569 loops=1)
Filter: (_c >= 30)
Total runtime: 42133.746 ms
See these :
-> Index Scan using "Nc" on structure (cost=0.00..105528.89 rows=26486 width=32) (actualtime=0.098..16.095 rows=734 loops=1)
-> Seq Scan on structure (cost=0.00..191619.56 rows=1765669 width=32) (actual time=8050.437..42117.062 rows=1569 loops=1)
In the index scan case, Planner thinks it'll get "rows=26486" but in reality only gets 734 rows.
In the seq scan case, Planner thinks it'll get "rows=1765669" but in reality only gets 1569 rows.
The two are way off-mark. 26486 still makes it choose an index scan because it's a small fraction of the table, but 1765669 is not.
Analyze, use more precise statistics (alter table set statistics), whatever... but you gotta get the planner correctly estimating these rowcounts.
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