On 01/25/2017 10:47 PM, John R Pierce wrote:
On 1/25/2017 12:59 PM, Thomas Kellerer wrote:
So here is my question: how does Postgres estimate/know the memory
needed for the aggregation? Or does it dynamically resize the memory
if the initial assumption was wrong?
my understanding is it fits as much as it can into a work_mem sized
allocation, and if thats not enough uses temporary files and multiple
passes.
That only works for hash joins, not for hash aggregates. Hash aggregate
is about the only operation in PostgreSQL that can cause OOM because of
under-estimation.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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