On Thu, Sep 22, 2011 at 2:51 PM, Tom Lane <tgl@sss.pgh.pa.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. If there
were any other op classes that had this abstract property of always
putting strings with common prefixes in a contiguous block then it
would continue to work without having to know where to find the
boundaries of that contiguous block.
Just as an example, if you had a pattern_ops opclass that sorted the
string assuming it was in some other encoding like, say, EBCDIC, then
make_greater_string would have to learn about it but Robert's model
would just work.
This isn't enitirely facetious. Sorting by EBCDIC ordering would be
silly but I vague recall there being some examples that wouldn't be
silly. And perhaps some collations could actually be marked as being
acceptable even if they don't sort in pure ascii ordering and
make_greater_string doesn't actually know about them.