performance bug in instrument.c

Hi,

it seems there's a issue in instrument.c that may have significant
performance impact for some plans when running explain analyze with
"TIMING OFF".

There's this piece of code in InstrStartNode:
   if (instr->need_timer && INSTR_TIME_IS_ZERO(instr->starttime))       INSTR_TIME_SET_CURRENT(instr->starttime);
else      elog(DEBUG2, "InstrStartNode called twice in a row");
 

but it should actually be like this
   if (instr->need_timer && INSTR_TIME_IS_ZERO(instr->starttime))       INSTR_TIME_SET_CURRENT(instr->starttime);
elseif (instr->need_timer)       elog(DEBUG2, "InstrStartNode called twice in a row");
 

or maybe
   if (instr->need_timer)   {       if (INSTR_TIME_IS_ZERO(instr->starttime))
INSTR_TIME_SET_CURRENT(instr->starttime);      else           elog(DEBUG2, "InstrStartNode called twice in a row");
}

The current code calls the "else" branch everytime when "TIMING OFF" is
used, and this may lead to serious performance degradation - see for
example this:
 CREATE TABLE x AS SELECT id FROM generate_series(1,15000) s(id); ANALYZE x; EXPLAIN ANALYZE SELECT a.id FROM x a, x
b;

Current code:
                              QUERY PLAN
-----------------------------------------------------------------------Nested Loop  (cost=0.00..2812971.50
rows=225000000width=4) (actual
 
time=0.013..29611.859 rows=225000000 loops=1)  ->  Seq Scan on x a  (cost=0.00..217.00 rows=15000 width=4) (actual
time=0.006..2.847 rows=15000 loops=1)  ->  Materialize  (cost=0.00..292.00 rows=15000 width=0) (actual
time=0.000..0.740 rows=15000 loops=15000)        ->  Seq Scan on x b  (cost=0.00..217.00 rows=15000 width=0)
(actual time=0.003..1.186 rows=15000 loops=1)Total runtime: 36054.079 ms
(5 rows)

and with the fix
                              QUERY PLAN
-----------------------------------------------------------------------Nested Loop  (cost=0.00..1458333.00
rows=116640000width=4) (actual
 
time=0.021..13158.399 rows=100000000 loops=1)  ->  Seq Scan on x a  (cost=0.00..153.00 rows=10800 width=4) (actual
time=0.014..1.960 rows=10000 loops=1)  ->  Materialize  (cost=0.00..207.00 rows=10800 width=0) (actual
time=0.000..0.499 rows=10000 loops=10000)        ->  Seq Scan on x b  (cost=0.00..153.00 rows=10800 width=0)
(actual time=0.003..1.037 rows=10000 loops=1)Total runtime: 16017.953 ms
(5 rows)

Obviously this is a quite extreme example, most plans won't be hit this
hard.

This was discovered by Pavel Stehule when backporting my "TIMING OFF"
patch (which introduced the bug).

regards
Tomas