What is the capability of the query optimizers in each case (Oracle 8i and SQL Server 2000)? Can programmers change the query plan?
I can only answer about Oracle 8i since I do not have too much experience with SQL Server. Hopefully, your question can be posed to a SQL Server expert.
Oracle 8i & 9i have come a long way in improving their Cost Based Optimizers. If at all possible, you should let the system's optimizer determine the best execution plan. But this means keeping your statistics up to date.
If you wish to control your SQL execution in Oracle, then you can use hints in your SQL statement. For instance:
SELECT /*+ INDEX(table_a_pk) */ col1, col2, col3 FROM table_a WHERE col1='somevalue';The hint appears between the '/*+' and '*/' characters. This hint tells the system to use the specified index. There are many more hints available which you might want to look at in the Oracle 8i Performance Tuning Guide.
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