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From: Moreno Andreo <[email protected]>
To: [email protected]
Subject: Re: Functions and Indexes
Date: Tue, 19 Nov 2024 14:30:37 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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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

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