"Merlin Moncure" <[EMAIL PROTECTED]> wrote ..
> select * from t where
>       a >= a1 and
>      (a >  a1 or b >= b1) and
>      (a >  a1 or b > b1 or c > c1)

I don't see why this is guaranteed to work without an ORDER BY clause, even if TABLE t 
is clustered on the correct index. Am I missing something? I have two suggestions: 

(1) I think I would have written 

(a >= a1 AND b>=b1 AND c>=c1) ORDER BY a,b,c LIMIT 1 OFFSET 1;

using the way LIMIT cuts down on sort time (I've never tried it with both LIMIT and 
OFFSET, though; you could always use LIMIT 2 and skip a record client-side if that 
works better).

(2) I've seen code where depending on the types and values of the fields, it was 
possible to construct a string from a, b, c by some sort of concatenation where the 
index now agreed with the lexicographic (dictionary) ordering on the string. Postgres 
could do that with a functional index, if your values can be used with this trick.

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