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
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