Am 17.07.12 05:21, schrieb Tom Lane:
Samuel Vogel <> writes:
I'm currently on a university research project if performance could be
increased by substituting different inter-node search algorithms instead
of the currently used binary search.
Hm, what have you got in mind exactly?

At first I will try a simple interpolation search, but problems start there since I need to have a numerical representation of the index keys (or map them to one) to do the interpolation.

But I'm having troubles understanding how the general b-tree
implementation (nbtree.h) is used to represent for example a simple
primary key on an integer column. I've debug printed the
'scankey->sk_argument' and all attributes of the index tuples on the
pages being traversed (simply ran 'DatumGetInt32' on both) but I never
see one of the integers actually appearing in my table being logged when
I do a select.
Not clear what you did wrong from this amount of detail, but integer
keys ought to be pretty obvious at the debugger level.

Okay, to be more specific: Printing 'DatumGetInt32(scankey->sk_argument)' in '_bt_compare' never shows me 50 when I execute this query: SELECT * FROM simpletest WHERE id = 50;

This is why I assume that all column values are hashed before they are
pushed into the b-tree,
PG's b-trees do not hash anything.  If you're not seeing interpretable
key values then you're doing something wrong in your inspection

Okay, how are indexes on char/text columns handled then? Are they hashed before being put into the b-tree or is my assumption correct, that in that case the Datum is only a link to where the actual data is stored and only 'scankey->sk_func' knows how to make use of it (compare it). In that case it would be extremly hard to get to a numeric representation which can be used for the interpolation.

Samuel Vogel

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