On 07/18/2018 06:57 AM, Thomas Kellerer wrote:
Adrian Klaver schrieb am 18.07.2018 um 15:06:
In the chapter "Database File layout" the pgsql_tmp is explained as follows:
Temporary files (for operations such as sorting more data than can fit in memory)
are created within PGDATA/base/pgsql_tmp, or within a pgsql_tmp subdirectory of
a tablespace directory
However the documentation for "temp_tablespaces" states:
Temporary files for purposes such as sorting large data sets are also created
in these tablespaces.
How do these two things related to each other?
Does this mean that if I do not explicitly create a dedicated "temp tablespace" then the pgsql_tmp subdirectory is used.
But _if_ I do create a temp tablespace (by creating one, and adding it to temp_tablespaces) then the sorting is done *there*?
Yes, for those objects that do not have a tablespace specified in their CREATE statement.
So far I thought that a temp tablespace is only used for temporary tables (and indexes on them) but that paragraph in the
temp_tablespaces documentation seems to indicate otherwise.
The Database File Layout section you quoted above says the same
thing. Basically setting temp_tablespaces just overrides where temp
objects and operation files are placed when a tablespace is not
specified in their creation.
Thanks.
I understand the relation between explicitly CREATEd objects and the temp tablespace(s).
But what about the (temp) space needed for e.g. sorting, grouping or intermediate results from CTEs or derived tables?
Is that also controlled through the temp_tablespaces?
Yes, all setting temp_tablespace from '' to some_tablespace(s) does is
redirect the creation of unspecified temp files from the db
default_namespace/pgsql_tmp to the named some_tablespace(s)/pgsql_tmp.
To verify this create a tablespace and add it to temp_tablespace and
then do temp operations and look at the_tablespace/pgsql_tmp/.
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
