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Reverse btree indexes
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* Reverse btree indexes
@ 2021-06-22 18:46  Zach Aysan <[email protected]>
  0 siblings, 1 reply; 3+ messages in thread

From: Zach Aysan @ 2021-06-22 18:46 UTC (permalink / raw)
  To: [email protected]

Hey there! Thanks for putting together such wonderful documentation. I have
a small suggestion for improvement.

*Page:* https://www.postgresql.org/docs/13/indexes-types.html

*Existing language:*

for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar'.


*Desired improvement:*

 for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
> which would require a reversed index on the field. Postgres will
> automatically use the reverse index for LIKE '%bar'


*Or if it doesn't:*


> for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
> which would require a reversed index on the field. To use the reversed
> index, query with reverse(col) like reverse('%bar').


I hope you all have a great day!

Zach


^ permalink  raw  reply  [nested|flat] 3+ messages in thread

* Re: Reverse btree indexes
@ 2021-06-22 22:51  David G. Johnston <[email protected]>
  parent: Zach Aysan <[email protected]>
  0 siblings, 1 reply; 3+ messages in thread

From: David G. Johnston @ 2021-06-22 22:51 UTC (permalink / raw)
  To: Zach Aysan <[email protected]>; +Cc: Pg Docs <[email protected]>

On Tue, Jun 22, 2021 at 2:28 PM Zach Aysan <[email protected]> wrote:

> *Desired improvement:*
>
>  for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
>> which would require a reversed index on the field. Postgres will
>> automatically use the reverse index for LIKE '%bar'
>
>
> *Or if it doesn't:*
>

It doesn't, otherwise the documentation wouldn't need to point out: but not
col LIKE '%bar' ...


>
>> for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
>> which would require a reversed index on the field. To use the reversed
>> index, query with reverse(col) like reverse('%bar').
>
>
>
This type of commentary isn't usually something we include in the
documentation...and I'm not too keen on "reversed index" as a phrase
regardless.

David J.


^ permalink  raw  reply  [nested|flat] 3+ messages in thread

* Re: Reverse btree indexes
@ 2021-06-23 16:08  Alvaro Herrera <[email protected]>
  parent: David G. Johnston <[email protected]>
  0 siblings, 0 replies; 3+ messages in thread

From: Alvaro Herrera @ 2021-06-23 16:08 UTC (permalink / raw)
  To: David G. Johnston <[email protected]>; +Cc: Zach Aysan <[email protected]>; Pg Docs <[email protected]>

On 2021-Jun-22, David G. Johnston wrote:

> On Tue, Jun 22, 2021 at 2:28 PM Zach Aysan <[email protected]> wrote:

> >> for example, col LIKE 'foo%' or col ~ '^foo', but not col LIKE '%bar',
> >> which would require a reversed index on the field. To use the reversed
> >> index, query with reverse(col) like reverse('%bar').
> >
> This type of commentary isn't usually something we include in the
> documentation...and I'm not too keen on "reversed index" as a phrase
> regardless.

Maybe we can add it as a parenthical comment.  Something like

: The optimizer can also use a B-tree index for queries involving the
: pattern matching operators LIKE and ~ if the pattern is a constant and
: is anchored to the beginning of the string — for example, col LIKE
: 'foo%' or col ~ '^foo', but not col LIKE '%bar' (it is possible to
: use an expressional index to support queries such as the latter; see
: Section 11.7).  However, [...]

Then add an example in 11.7 Indexes on Expressions below the two
existing examples.  I think this is a noteworthy use of expressional
indexes; I've had to explain the reverse() idea a couple of times.

-- 
Álvaro Herrera                            39°49'30"S 73°17'W
"The eagle never lost so much time, as
when he submitted to learn of the crow." (William Blake)






^ permalink  raw  reply  [nested|flat] 3+ messages in thread

end of thread, other threads:[~2021-06-23 16:08 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2021-06-22 18:46 Reverse btree indexes Zach Aysan <[email protected]>
2021-06-22 22:51 ` David G. Johnston <[email protected]>
2021-06-23 16:08   ` Alvaro Herrera <[email protected]>

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