pgsql: Release memory allocated by dependency_degree

Release memory allocated by dependency_degree

Calculating degree of a functional dependency may allocate a lot of
memory - we have released mot of the explicitly allocated memory, but
e.g. detoasted varlena values were left behind. That may be an issue,
because we consider a lot of dependencies (all combinations), and the
detoasting may happen for each one again.

Fixed by calling dependency_degree() in a dedicated context, and
resetting it after each call. We only need the calculated dependency
degree, so we don't need to copy anything.

Backpatch to PostgreSQL 10, where extended statistics were introduced.

Backpatch-through: 10
Discussion: https://www.postgresql.org/message-id/20210915200928.GP831%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ad8a166ca86846ab691bd6dafc695e0f7dd96012

Modified Files
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src/backend/statistics/dependencies.c | 22 ++++++++++++++++------
1 file changed, 16 insertions(+), 6 deletions(-)