Обсуждение: best way to retreive the next record in a multi column index
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
On Fri, Aug 15, 2003 at 13:42:23 -0400, Merlin Moncure <merlin.moncure@rcsonline.com> wrote: > > 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. > How about something like the following: select * from t where a >= a1 and b >= b1 order by a, b limit 1 offset 1;
Bruno Wolff III wrote: > How about something like the following: > select * from t > where a >= a1 and b >= b1 > order by a, b limit 1 offset 1; Well, this may have recently changed, but the offset clause is not suitable for arbitrary jumps over large tables. Essentially, pg does an index lookup to the first element then sequential scans until the offset criteria is met. Even if that was not the case there is another problem: Suppose while you are iterating over your table another backend deletes a row after your initial start position; this will cause a record to get skipped! (unless inside a transaction, of course, but that can't be assumed). I also spent a lot of time thinking about use some type of concatenation and functional indices to get around the multi column issue (then things would be really simple!). This turned out to be a very complicated and I ended up giving it up: I was stymied in the creation of a 'universal concatenation' function, plus losing the elegant syntax to do partials was a loss. Merlin