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Like the title says, if anyone has the answer I would like to know. I’ve been googling but couldn’t find a straight answer.
Example:
This works
SELECT COUNT(*) FROM Table1 TB1, Table2 TB2
WHERE TB1.Field1 = TB2.Table2
This seems to take hours
SELECT COUNT(*) FROM Table1 TB1, Table2 TB2
WHERE TB1.Field1 <> TB2.Table2
It’s important to realize that SQL is a declarative language and not an imperative one. You describe what conditions you want your data to fit and not how those comparisons should be executed. It’s the job of the database to find the fastest way to give you an answer (a task taken over by the query optimizer). This means that a seemingly small change in your query can result in a wildly different query plan, which in turn results in a wildly different runtime behaviour.
The
=comparison can be converted to and optimized the same way as a simple join on the two fields. This means that normal indices can be used to execute the query very fast, probably without reading the actual data and using only the indices instead.A
<>comparison on the other hand requires a full cartesian product to be calculated and checked for the condition, usually (there might be a way to optimize this with the correct index, but usually an index won’t help here). It will also usually return a lot more results, which adds to the execution time.