[PATCH] Remove unncessary localtime() calls during data type conversion

Hi,

While doing some profiling work on an ODBC client, we noticed tons
of libc localtime() and __tz_convert() calls on a hot code path
(SQLFetch()), but most of the fields being retrieved were not of
date/time type.


These libc calls are costly (non-trivial work in terms of string ops,
timzone conversion etc.) and the cost adds up quickly for high-frequency
call like SQLFetch(). We noticed that this penalty is paid even
for data types which aren't date/time related. Code inspection
showed localtime() call in convert.c:copy_and_convert_field()
at function start, being invoked regardless of whether output of
localtime() is used or not.

Attached is a non-intrusive patch that moves the call down
to where it's needed (for both getting data from server and
sending data to server), but I'm not sure if the patch is complete
(it covers the case where source data type is time but
destination needs date part, "TIME->DATE[TIME]").

Could you please review this patch and modify it for correctness
and consider for committing it?

---

A longer term cleaner approach would be to refactor the code to aim for
at most one call to localtime() (e.g. if it's already invoked once
in copy_and_convert_field() before, reuse that data). However
that is much more intrusive change.

I checked the test/src/result-conversions-test.c code and the
test case specific to this scenario is commented out (since it
the expected result would depend on actual current date).
I un-commented those test cases, ran the test with and without
my patch and got same results. Also the data looks ok for
"TIME->DATE[TIME]" case, input is time, output is date.

   1114a1176
   > '13:23:34' (time) as SQL_C_TYPE_DATE: y: 2015 m: 6 d: 12

2015-6-12 is indeed current date.

Thanks,
  Nikhil