Re: Functions and Indexes

On 19/11/24 12:34, Laurenz Albe wrote:
> On Tue, 2024-11-19 at 11:53 +0100, Moreno Andreo wrote:
>>>> What about if query becomes
>>>> SELECT foo1, foo2 FROM bar WHERE (POSITION(foo1 IN 'blah blah') >0)
>>> You could create an index like
>>>
>>>     CREATE INDEX ON bar (position(foo1 IN 'blah blah'));
>>>
>>> Alternatively, you could have a partial index:
>>>
>>>     CREATE INDEX ON bar (foo1) INCLUDE (foo2)
>>>     WHERE position(foo1 IN 'blah blah') > 0;
>> Interesting. Never seen this form, I'll look further on it.
>>
>> I stumbled into
>> https://www.cybertec-postgresql.com/en/indexing-like-postgresql-oracle/
>> and discovered text_pattern_ops.
>> I'm wondering if it can be of any use in my index, that should hold a
>> WHERE condition with a combination of LIKE and the POSITION expression
>> above.
>> More docs to read ... :-)
> I don't think "text_pattern_ops" will help here - queries that use LIKE
> to search for a substring (LIKE '%string%') cannot make use of a b-tree
> index.
Oh, OK, i was happy to use BTREEs 'cause I had some issues with GIN/GIST
(like indexes way bigger than table and so inefficient). OK, I'll stick
with these and try harder to obtain better results.

One thing I can't understand well.
In
https://www.cybertec-postgresql.com/en/join-strategies-and-performance-in-postgresql/
you say
"Note that for inner joins there is no distinction between the join
condition and the|WHERE|condition, but that doesn't hold for outer joins."
What do you mean?

Thanks
Moreno