Can anybody help me with this? (sorry
for posting on hackers) I need to be able determine the next row based on a non
unique key (index). I have solved
this problem, but I would like to know if there is a simpler solution. For those of you who have ever dealt
with COBOL, this is an on the fly sql construction of a 'READ NEXT' statement
following a START. Very similar to
cursors, but because of the transactional limitations of cursors they cannot be
used in this context. Example: I have a table t with columns a, b, c. I have values a1, b1, c1 for those
columns and would like to know the next value in the table when ordered by a,
b. I have values a1, b1, and oid1
and would like to find the very next record in the table (essentially looking
for the next record in the index). I have two solutions: one with 'or' logic and one with 'and'
logic. Note: if the index we are
scanning has the unique constraint, the oid part of
the logic (and the index) can be left out. create index t_idx on t(a, b, oid); *or* logic: select * from t where a
> a1 OR (a
= a1 and b > b1) OR (a
= a1 and b = b1 and oid > oid1) order by a, b, oid *and* logic select * from t where a
>= a1 AND (a
> a1 or b >= b1) AND (a
> a1 or b > b1 or oid > oid1) order by a, b, oid I think, of the two, the or logic
is much better. The problem with
both approaches is that when we have a 4 column based key (common in COBOL) our
index is based on a,b,c,d,o and the number of
comparisons (and our select statement) becomes large, and performance is very
important! If some logical genius
knows how to reduce the above logic into a more direct approach, feel free to
comment. Postgres properly optimizes both cases, and uses the key
even for a table with 1 million + records in it, the
answer comes back right away. My question is: is there a simpler way to do this? AFIK
there is no way in sql to directly find the 'next' or 'previous' record in an
ordered index (in other words, I have oid n, what is
the next oid in the index?) without using the above
logic. In other words, I am missing
the ability to deal with a multi column index value in a comparison as a single
entity. p.s. the above
queries are 'sliding window' queries similar to cursors. If your table traversal can be defined
by an (unique) index, you can use the above templates to slide over the tables
without the use of a cursor. Merlin |
- Re: [HACKERS] best way to retreive the next record in a mu... Merlin Moncure
- Re: [HACKERS] best way to retreive the next record in... Bruno Wolff III
- Re: [HACKERS] best way to retreive the next record in... Merlin Moncure