(In most cases, Oracle uses indexes to traverse tables faster, while the optimizer mainly improves performance based on defined indexes. However, if the SQL code written in the where clause of the SQL statement is unreasonable, the optimizer will delete the index and use the full table scan. Generally, this kind of SQL statement is called inferior SQL statement. When writing SQL statements, we should be clear about the optimizer's principle of deleting indexes, which helps to write high-performance SQL statements.)
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