Actually,
the cardinality detection sends three SELECT COUNT -statements to the server. I don’t remeber the actual statements, but something like: 1) SELECT COUNT(DISTINCT Table1.MyColumn) 2) SELECT COUNT(DISTINCT Table2.OtherColumn) 3) SELECT COUNT(*) FROM Table1, Table2 WHERE Table1.MyColumn=Table2.OtherColumn
Then some comparison is made based on the results of the three statements.
I may be a bit off with the actual statements, but three COUNTs are sent, however. Obviously, this can be a real killer on bigger databases. That’s why I always uncheck the “Check Cardinality” option in the “Check Integrity” dialog.
-Harri
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