On Wed, 15 Jan 2025 14:40:19 -0800
Jeff Davis <pgsql@j-davis.com> wrote:
> My apologies, I sent the previous email prematurely. Let me try again:
>
> On Wed, 2025-01-15 at 14:34 -0800, Jeff Davis wrote:
> > On Wed, 2025-01-15 at 01:40 +0900, Yugo NAGATA wrote:
> > > > > For example, "t ~~ '123foo%'" is converted to "(t >= '123foo'
> > > > > AND
> > > > > t < '123fop')"
> > > > > and index scan can be used for this condition. On the other
> > > > > hand,
> > > > > "t ~~* '123foo'"
> > > > > cannot be converted and sequential scan is used.
> > > > >
> > > > > Even in this case, we can use a bitmap index scan for the
> > > > > condition
> > > > > "(t >= '123f' AND t < '123g') OR "(t >= '123F' AND t < '123G')"
> > > > > followed by
> > > > > recheck by the original condition "t ~~* '123foo'", and this
> > > > > could be faster
> > > > > than seqscan.
>
> In theory, there could be many OR clauses:
>
> (t >= '123foo' AND t < '123fop') OR
> (t >= '123Foo' AND t < '123Fop') OR
> (t >= '123fOo' AND t < '123fOp') OR
> (t >= '123FOo' AND t < '123FOp') OR
> ...
>
> How should that be limited?
Instead of generating complete patterns considering every case-varying characters,
two clauses considering only the first case-varying character are generated.
For example, for the condition "t ILIKE '123foo%'", the generated condition is
"(t >= '123f' AND t < '123g') OR "(t >= '123F' AND t < '123G')".
Rows meeting "(t >= '123f' AND t < '123g')" includes those whose "t" start with
'123f', that is meeting the following;
> (t >= '123foo' AND t < '123fop') OR
> (t >= '123fOo' AND t < '123fOp') OR
> ...
, and rows meeting "(t >= '123F' AND t < '123G')" includes those whose "t" start
with '123F', that is meeting the following;
> (t >= '123Foo' AND t < '123Fop') OR
> (t >= '123FOo' AND t < '123FOp') OR
> ...
It is required that the second case-varying character and later are checked after
the index scan, and some rows are filtered, but it will be still faster than full
sequential scan.
Regards,
Yugo Nagata
--
Yugo NAGATA <nagata@sraoss.co.jp>
