I was wondering whether it is possible to teach the planner to handle
DISTINCT in a more efficient way:
em=# explain select distinct lastname from import.testtest;
QUERY PLAN
--------------------------------------------------------------------------------
Unique (cost=2647377.45..2709467.70 rows=1 width=7)
-> Sort (cost=2647377.45..2678422.58 rows=12418051 width=7)
Sort Key: lastname
-> Seq Scan on testtest (cost=0.00..370082.51 rows=12418051
width=7)
(4 Zeilen)
Isn't it possible to perform the same operation using a HashAggregate?
We have seen that a GROUP BY workaround is usually a lot faster than
sort->unique - at least when work_mem is large enough.
best regards,
hans
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