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Given a query q to execute. Is there a way to determine how much disk-space postgresql will (or used) as temporary_space in order to execute it (during its execution) ?

I'm under Ubuntu 12.0.4 with postgresql 9.1

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Knowing how much will be used is hard. How much has been used, you there are logging options for that.

set log_temp_files=0;
set client_min_messages TO log;
select count(distinct x) from generate_series(1,10000000) x;

LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp25126.2", size 120127488.

LOG: temporary file: path "base/pgsql_tmp/pgsql_tmp25126.1", size 140000000

1

You can use EXPLAIN (ANALYZE, BUFFERS) SELECT ... to see how much space your current query with current params take. But it may need different amounth of space for different input parameters or different data set.

Some examples where the temp files are used:

Sort Method: external merge  Disk: 6848kB
Buffers: shared hit=4428, temp read=2305 written=2305

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