pgsql: Get rid of copy_partition_key

Get rid of copy_partition_key

That function currently exists to avoid leaking memory in
CacheMemoryContext in case of trouble while the partition key is being
built, but there's a better way: allocate everything in a memcxt that
goes away if the current (sub)transaction fails, and once the partition
key is built and no further errors can occur, make the memcxt permanent
by making it a child of CacheMemoryContext.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20171027172730.eh2domlkpn4ja62m@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8a0596cb656e357c391cccf12854beb2e05f3901

Modified Files
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src/backend/utils/cache/relcache.c | 71 +++++---------------------------------
1 file changed, 9 insertions(+), 62 deletions(-)