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    <p>If everything works as it should it shouldn't, BUT we all know everything doesn't work the way it should especially when it comes to the query optimizer, query plan caching and statistics.</p> <p>First I would suggest rebuilding index and statistics, then clearing the query plan cache just to make sure that's not screwing things up. However I've experienced problems even when that's done.</p> <p>I've experienced some cases where a left join has been faster than a inner join.</p> <p>The underlying reason is this: If you have two tables and you join on a column with an index (on both tables). The inner join will produce the same result no matter if you loop over the entries in the index on table one and match with index on table two as if you would do the reverse: Loop over entries in the index on table two and match with index in table one. The problem is when you have misleading statistics, the query optimizer will use the statistics of the index to find the table with least matching entries (based on your other criteria). If you have two tables with 1 million in each, in table one you have 10 rows matching and in table two you have 100000 rows matching. The best way would be to do an index scan on table one and matching 10 times in table two. The reverse would be an index scan that loops over 100000 rows and tries to match 100000 times and only 10 succeed. So if the statistics isn't correct the optimizer might choose the wrong table and index to loop over.</p> <p>If the optimizer chooses to optimize the left join in the order it is written it will perform better than the inner join.</p> <p>BUT, the optimizer may also optimize a left join sub-optimally as a left semi join. To make it choose the one you want you can use the force order hint.</p>
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