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When joining two tables, for each row in one table Oracle looks up the matching rows in the other table.Consider the following SQL Trace output from TKProf.I was at the supermarket the other day waiting my turn at the checkout behind another guy.The checkout-chick (I'm sure there is a more PC term, I just don't know what it is) just finished scanning his groceries and he asked her to wait until his wife returned with a few last-minute items.The Rule Based Optimiser is stupid however; one of its rules is to use an index if it is available.This problem will fix itself when we remove the hint.Although the optimiser is very reliable on simple queries such as this, the real world contains much more complex queries that are beyond the capability of the CBO to optimise.In some cases the CBO will wrongly believe that a high-volume join is actually low-volume, and wrongly choose a Nested Loops join.
It is the cause of almost every poorly tuned high-volume SQL I have ever seen.
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Hash joins are the preferred method for large data volumes, because their nested operations are performed , not on disk, and they scale better than MERGE joins.
Interestingly, Oracle was right to choose the MERGE join for the small volumes; note that MERGE performed only 11 buffer reads, whilst HASH performed 15, and NESTED LOOPS performed 26 - the worst of all.Consider a sub-optimal SQL (SQL1) that takes 10 minutes to run and its optimal tuned equivalent (SQL2) that takes 1 minute to run.They perform the same logic and return the same results, but one is faster than the other.I've done it before, so steam didn't start coming out of my ears - yet.