pgsql: ALTER TABLE ADD COLUMN exhibits a significant memory leak when

Log Message:
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ALTER TABLE ADD COLUMN exhibits a significant memory leak when adding a
column with a default expression. In that situation, we need to rewrite
the heap relation. To evaluate the new default expression, we use
ExecEvalExpr(); however, this can allocate memory in the current memory
context, and ATRewriteTable() does not switch out of the active portal's
heap memory context. The end result is a rather large memory leak (on
the order of gigabytes for a reasonably sized table).

This patch changes ATRewriteTable() to switch to the per-tuple memory
context before beginning the per-tuple loop. It also removes an explicit
heap_freetuple() in the loop, since that is no longer needed.

In an unrelated change, I noticed the code was scanning through the
attributes of the new tuple descriptor for each tuple of the old table.
I changed this to use precomputation, which should slightly speed up
the loop.

Thanks to steve(at)deefs(dot)net for reporting the leak.

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/src/backend/commands:
tablecmds.c (r1.142.4.1 -> r1.142.4.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.142.4.1&r2=1.142.4.2)