Fix removal of files in pgstats directories

Instead of deleting all files in stats_temp_directory and the permanent
directory on a crash, only remove those files that match the pattern of
files we actually write in them, to avoid possibly clobbering existing
unrelated contents of the temporary directory.  Per complaint from Jeff
Janes, and subsequent discussion, starting at message
CAMkU=1z9+7RsDODnT4=cDFBRBp8wYQbd_qsLcMtKEf-oFwuOdQ@mail.gmail.com

Also, fix a bug in the same routine to avoid removing files from the
permanent directory twice (instead of once from that directory and then
from the temporary directory), also per report from Jeff Janes, in
message
CAMkU=1wbk947=-pAosDMX5VC+sQw9W4ttq6RM9rXu=MjNeEQKA@mail.gmail.com
Be more wary of unwanted whitespace in pgstat_reset_remove_files().

sscanf isn't the easiest thing to use for exact pattern checks ...
also, don't use strncmp where strcmp will do.
release notes: update link to 9.3 PL/pgSQL constraint error info

Backpatch to 9.3.

Pavel Stehule
Merge branch 'master' into returning_before_after
Merge branch 'returning_before_after' of github.com:davidfetter/returning_before_after into returning_before_after
Unconditionally use the WSA equivalents of Socket error constants.

This change will only apply to mingw compilers, and has been found
necessary by late versions of the mingw-w64 compiler. It's the same as
what is done elsewhere for the Microsoft compilers.

If this doesn't upset older compilers in the buildfarm, it will be
backpatched to 9.1.

Problem reported by Michael Cronenworth, although not his patch.
docs: Remove second 'trim' index reference

Per suggestion from Vik Fearing
Fix hash table size estimation error in choose_hashed_distinct().

We should account for the per-group hashtable entry overhead when
considering whether to use a hash aggregate to implement DISTINCT.  The
comparable logic in choose_hashed_grouping() gets this right, but I think
I omitted it here in the mistaken belief that there would be no overhead
if there were no aggregate functions to be evaluated.  This can result in
more than 2X underestimate of the hash table size, if the tuples being
aggregated aren't very wide.  Per report from Tomas Vondra.

This bug is of long standing, but per discussion we'll only back-patch into
9.3.  Changing the estimation behavior in stable branches seems to carry too
much risk of destabilizing plan choices for already-tuned applications.
In locate_grouping_columns(), don't expect an exact match of Var typmods.

It's possible that inlining of SQL functions (or perhaps other changes?)
has exposed typmod information not known at parse time.  In such cases,
Vars generated by query_planner might have valid typmod values while the
original grouping columns only have typmod -1.  This isn't a semantic
problem since the behavior of grouping only depends on type not typmod,
but it breaks locate_grouping_columns' use of tlist_member to locate the
matching entry in query_planner's result tlist.

We can fix this without an excessive amount of new code or complexity by
relying on the fact that locate_grouping_columns only gets called when
make_subplanTargetList has set need_tlist_eval == false, and that can only
happen if all the grouping columns are simple Vars.  Therefore we only need
to search the sub_tlist for a matching Var, and we can reasonably define a
"match" as being a match of the Var identity fields
varno/varattno/varlevelsup.  The code still Asserts that vartype matches,
but ignores vartypmod.

Per bug #8393 from Evan Martin.  The added regression test case is
basically the same as his example.  This has been broken for a very long
time, so back-patch to all supported branches.
Don't crash when pg_xlog is empty and pg_basebackup -x is used

The backup will not work (without a logarchive, and that's the whole
point of -x) in this case, this patch just changes it to throw an
error instead of crashing when this happens.

Noticed and diagnosed by TAKATSUKA Haruka
Account better for planning cost when choosing whether to use custom plans.

The previous coding in plancache.c essentially used 10% of the estimated
runtime as its cost estimate for planning.  This can be pretty bogus,
especially when the estimated runtime is very small, such as in a simple
expression plan created by plpgsql, or a simple INSERT ... VALUES.

While we don't have a really good handle on how planning time compares
to runtime, it seems reasonable to use an estimate based on the number of
relations referenced in the query, with a rather large multiplier.  This
patch uses 1000 * cpu_operator_cost * (nrelations + 1), so that even a
trivial query will be charged 1000 * cpu_operator_cost for planning.
This should address the problem reported by Marc Cousin and others that
9.2 and up prefer custom plans in cases where the planning time greatly
exceeds what can be saved.
Merge branch 'master' into returning_before_after
Merge branch 'returning_before_after' of github.com:davidfetter/returning_before_after into returning_before_after