Things SQL needs: MERGE JOIN that would seek | Hacker News Reader

Things SQL needs: MERGE JOIN that would seek

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I think it would be better still to, when seeking forward, have it examine a couple of records, then start jumping forward in powers of 2 until it has located an interval with the next value in it, which it then does a binary search on to find the right location.

This is at most a factor of 2 less efficient than a binary search, it is fast when the sequential reads are fast, and it doesn't rely on statistics telling the whole story. (For instance the size discrepancy could be explained by a small number of large gaps. This strategy would quickly jump the gaps while still getting fast sequential read behavior elsewhere.)

Unfortunately, with a B+Tree index, you cannot jump in powers of 2 (well, you can but only within one page). To find a record with a given offset you'd need to traverse the whole linked list of the leaf-level pages, and that's what MERGE JOIN does by default.

However, this solution would be nice if you had a sorted recordset which allows efficient positional seek (for instance, a materialized result of the previous operation which required sorting).

He's cheating a bit. He created two tables each with 1 million rows, offset by 999,000. (i.e. 1 - 1,000,000 and 999,000 - 1,999,000)

Which of course is the best case scenario for this query.

But this is very unlikely in the real world. In the real world you use sequential IDs, and almost all the IDs exist. So seek is not useful.

If you use random IDs (like using a GUID for a primary key), it's still not useful. If your random number generator is at all good your index numbers are pretty well distributed, without the large gaps this query likes.

> He's cheating a bit. He created two tables each with 1 million rows, offset by 999,000. (i.e. 1 - 1,000,000 and 999,000 - 1,999,000)

Of course this is cheating, but only to demonstrate the point :)

> But this is very unlikely in the real world. In the real world you use sequential IDs, and almost all the IDs exist. So seek is not useful.

Yes, and that's why the HASH JOIN is almost always used instead by the real optimizers :)

However, there are situations when the gaps are expected. Say, the right recordset is small compared to the left one. Usually, a NESTED LOOPS is used in this case with the right recordset leading, but each loop uses a full-fledged seek from the top of the B+Tree which can be inefficient when the right recordset is not so small.

Using a statistics-assisted MERGE JOIN in this case would help to mix the seeks and the scans.

Say, if the statistics say "current right is 500, current left is 300, there are 40000 records from 0 to 1000 in the left recordset", we know that most probably we'll need to skip 8000 records to reach the right pointer and we better seek.

But if on next step it will say "current right is 501, current left is 500, there are 40000 records from 0 to 1000 in the left recordset", we know that there are only 40 records left which are most probably in one page and we can scan instead.

It can be thought of as an improved NESTED LOOPS which caches the current position.

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