Обсуждение: [GENERAL] hard parse?
Hi
I have a query where a filter would always be negative, how many steps, out these:
- parsing and syntax check
- semantic analysis
- transformation process (query rewrite based on system or user-defined rules)
- query optimization
- execution
would be performed or not? Also, where in the documentation can I found out which of the above phases would be performed?
For example, for a query such as the following:
select x,y from test1 where 1=0;
Thanks
P
HiI have a query where a filter would always be negative, how many steps, out these:
- parsing and syntax check
- semantic analysis
- transformation process (query rewrite based on system or user-defined rules)
- query optimization
- execution
would be performed or not? Also, where in the documentation can I found out which of the above phases would be performed?For example, for a query such as the following:select x,y from test1 where 1=0;
I'm inferring behavior here but...
All of them. You are still going to get a result set with zero records and the correct column structure. i.e., "Execution". None of the other stuff can be skipped in getting to engine to that point. With a "always false" predicate and that simple of a query structure most of the other stuff, including execution, is probably performed is seemingly zero time but it still has to work through that step of the process - if nothing else than to move through an if-branch to decide that nothing material needs to be done.
David J.
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Thu, Sep 21, 2017 at 5:48 AM, Peter Koukoulis <pkoukoulis@gmail.com>
> wrote:
>> I have a query where a filter would always be negative, how many steps,
>> out these:
>>
>> - parsing and syntax check
>> - semantic analysis
>> - transformation process (query rewrite based on system or
>> user-defined rules)
>> - query optimization
>> - execution
>>
>> would be performed or not? Also, where in the documentation can I found
>> out which of the above phases would be performed?
> All of them.
Yeah. The question is more usefully formulated as "how much will the
query optimizer collapse a query with a constant-false condition"?
You can answer that with EXPLAIN, eg.
regression=# create table test1 (x int, y int);
CREATE TABLE
regression=# explain select x,y from test1 where 1=0; QUERY PLAN
------------------------------------------Result (cost=0.00..0.00 rows=0 width=8) One-Time Filter: false
(2 rows)
In this case the answer is "pretty far" --- you get a valid but
dummy plan, which will just exit without returning any rows.
regards, tom lane
--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general