space for optimalization: DISTINCT without index - Mailing list pgsql-hackers

Hello

I did some test and I can see so DISTINCT works well on indexed columns, but 
is slow on derived tables without indexes. If I use without distinct group 
by I get much better times.

SELECT DISTINCT a, b FROM tab
SELECT a,b FROM tab GROUP BY a, b.

Can You Explain it.

Thank You
Pavel Stehule

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