Re: b-tree index search algorithms

Am 17.07.12 19:38, schrieb Tom Lane:
> btree knows nothing about the datatypes it's working on except that
> they have comparison functions. Converting the values to some sort of
> numeric scale that you can interpolate on seems logically dubious and
> fraught with practical difficulties. Now, we do have some code in
> selfuncs.c that tries to do that, for some data types, but it's only
> for planner estimation purposes, and we don't rely very heavily on its
> results even in that context. Depending on it to be right for search
> purposes sounds pretty scary.

'convert_string_to_scalar' and others look interesting in selfuncs.c,
thanks for the pointer!

>> Okay, how are indexes on char/text columns handled then?
> The datum values will be pointers to strings.

I can simply dereference it and read all bytes until a null-byte appears
(depending on the collation and that I know that it actually is a string)?

> The btree code is (or reasonably can be) aware that such values are
> pass-by-reference, and how to get to the bits. But the comparison
> semantics of two different values are not something it knows about
> except by asking the comparison function. This can be quite a
> nontrivial matter even for text, since we follow strcoll() comparison
> rules. regards, tom lane

How would the b-tree know exactly that a value is only a reference? And
even in that case you say that it could get the bits, but make no use of
it, since it does not know what they represent, right?

Regards,
Samuel