pgsql: Fix handling of pgstat counters for TRUNCATE in a prepared trans

Fix handling of pgstat counters for TRUNCATE in a prepared transaction.

pgstat_twophase_postcommit is supposed to duplicate the math in
AtEOXact_PgStat, but it had missed out the bit about clearing
t_delta_live_tuples/t_delta_dead_tuples for a TRUNCATE.

It's harder than you might think to replicate the issue here, because
those counters would only be nonzero when a previous transaction in
the same backend had added/deleted tuples in the truncated table,
and those counts hadn't been sent to the stats collector yet.

Evident oversight in commit d42358efb.  I've not added a regression
test for this; we tried to add one in d42358efb, and had to revert it
because it was too timing-sensitive for the buildfarm.

Back-patch to 9.5 where d42358efb came in.

Stas Kelvich

Discussion: <EB57BF68-C06D-4737-BDDC-4BA778F4E62B@postgrespro.ru>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3cd504254f604a37adefc2bd9d43a0a6041463fd

Modified Files
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src/backend/postmaster/pgstat.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)