Like for the previous patches it seems that there is
no obvious performance degradation too on regular queries (according
to
pgbench).
pgbench probably isn't a very good test for this sort of thing - it
only issues very short-running queries where the cost of evaluating
expressions is a relatively small part of the total cost. Even if
things get worse, I'm not sure if you'd see it.
If there's a mistake, for example, more than 1 try to replace cached
expressions in the whole query tree, results of "in buffer test" or
"mostly cache test" can different a little..
I'm not sure exactly
how you could construct a test case that could be harmed by this patch
- I guess you'd want to initialize lots of CacheExprs but never make
use of the caching usefully?
As I mentioned in the first letter about this feature it will be useful
for such text search queries [1]:
SELECT COUNT(*) FROM messages WHERE body_tsvector @@
to_tsquery('postgres');
And I'm not sure that it is logical to precalculate stable and immutable
functions themselves, but not to precalculate expressions that behave
like stable/immutable functions; precalculate some types of operators
and not to precalculate others (ScalarArrayOpExpr, RowCompareExpr). My
patch solves the problem that not all nodes are simplified in
eval_const_expressions_mutator (for example, ScalarArrayOpExpr) and
consts of other types now behave more like ordinary consts (for example,
composite types, coerce expressions, ConvertRowtypeExpr).
It could also be useful to test things like TPC-H to see if you get an
improvement.
I saw the examples of queries in TPC-H tests. If I'm not wrong they are
not the target tests for this functionality (nothing will be
precalculated). But it's a good idea to check that there's no a
performance degradation on them too.
[1]
https://www.postgresql.org/message-id/ba261b9fc25dea4069d8ba9a8fcadf35%40postgrespro.ru
--
Marina Polyakova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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