Thomas Munz wrote:
Hello List!
I tried today to optmize in our companies internal Application the
querys. I come to a point where I tried, if querys with LIMIT are slower
then querys without limit
I tried following query in 8.2.4. Keep in mind that the table hs_company
only contains 10 rows.
Probably too small to provide useful measurements.
ghcp=# explain analyze select * from hs_company; explain analyze select
* from hs_company limit 10;
Total runtime: 0.102 ms
Total runtime: 0.138 ms
1. I'm not sure the timings are accurate for sub-millisecond values
2. You've got to parse the LIMIT clause, and then execute it (even if it
does nothing useful)
I runned this query about 100 times and always resulted, that this query
without limit is about 40 ms faster
That's 0.4ms
Now I putted the same query in the file 'sql.sql' and runned it 100
times with:
psql test testuser -f sql.sql
Total runtime: 0.200 ms
Total runtime: 0.153 ms
The querys are equal but has different speeds. Can me someone explain
why that is?
Same as above - you've got to parse & execute the limit clause. There's
no way for the planner to know that the table has exactly 10 rows in it
at the time it executes.
--
Richard Huxton
Archonet Ltd
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