Re: [v9.2] make_greater_string() does not return a string in some cases

Greg Stark <stark(at)mit(dot)edu> writes:
> On Thu, Sep 22, 2011 at 2:51 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The essential problem here is "when can you stop scanning,
>> given a pattern with this prefix?", and btree doesn't know any more
>> about that than make_greater_string does; it would in fact have to use
>> make_greater_string or something isomorphic to it.

> Hm, as long as btree_pattern_ops is the only opclass that behaves this
> way that's more or less true. But Robert's right that if btree just
> stops when it finds something that doesn't match it doesn't need to
> hard code any knowledge of what the "next" value would be.

But you've added mechanism (and hence cycles) to btree searches,
and *you haven't actually gained anything*. If the feature is
restricted to only work for sort orderings in which common-prefix
strings are contiguous, then it doesn't do anything we can't do
just as well with the existing mechanism. Moreover, you'll still
need make_greater_string because of the problem of trying to extract
LIKE selectivity estimates from locale-dependent pg_statistic data.

regards, tom lane