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Dileep Dasa
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If there are no indexes on a table, then SQL uses default index which is on ROWID. If there are multiple indexes defined on the table, then as ChUIMonster said, SQL uses a cost based optimizer to select the best index. From SQL Development guide: "Selecting an index When the list of candidate index predicates has been determined, the optimizer selects which, if any, it will use for an index scan operation. This selection is cost-based. The optimizer computes the cost for each of the index candidates and the cost for a table scan using the default index. The candidate with the lowest cost is chosen."
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