"David G. Johnston" <[email protected]> writes:
> On Thu, Sep 21, 2017 at 5:48 AM, Peter Koukoulis <[email protected]>
> 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
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