I am experimenting with optimising a SQL statement. One version uses 4 LEFT JOIN’s and a 5-way CASE statement in the body. The second moves the filtering into the JOIN section, and I end up with 16 LEFT JOIN’s and no CASE statements.
My test involves selecting a single row. Both versions work. The first version takes 0.06 seconds. The second takes 0.23 seconds. On further experimentation, the time for the second one seems to taken in setting up the joins, because if I omit selecting anything from the joined tables, it still takes 0.23 seconds.
Exactly the same exercise on Sql Server results in 0.06 seconds for both versions.
I realise that, if I was selecting a large number of rows, 0.23 seconds is trivial and the overall result could be different. But still, it seems odd.
> Just out of curiosity, what if you PREPARE the statement, and take multiple timings?
My setup is a bit complicated, as I am executing the commands from a python program on Windows against a PostgreSQL database on Fedora, so I hope I did it correctly!
With that caveat, the results are that the time was reduced from 0.23 seconds to 0.22 seconds. The difference is consistent, so I think it is real.