Oracle update sql not using index

Why doesn't Oracle use my index? I verified that the index exists, but I cannot seem to force Oracle to use the index. Why does Oracle ignore an index? How do you force an index to be used? Answer: The Oracle cost-based optimizer carefully evaluates every query when making the decision whether to invoke a full-table scan or index access, and you can force index usage with in index hint. The goal of the SQL optimizer is to only force an index when it's the "best" access plan, given your optimization goals:.

Hello, Chris. Could you please elaborate why 53, rows is a lot for a 6 million rows table? May 15, - pm UTC. Your table has some number and date columns - let's say each row is about 50 bytes long. So that would be calls. That's an interesting explanation, but I still find hard to see such situation in real life. Somebody would have to put much effort to have about 53k rows evenly split on 53k differents blocks, like almost a manual job.

It would do so only if you explicitly set this parameter to a large value" I'm not questioning the explanation, in fact I'm just delving into the subject because I'm experiencing a similar situation as the original question: a 18 million rows table that refuses to use an optimal index when returning more than K rows.

When hinting the index for a more than k rows predicate I see the cost higher than a FTS. But it's curious to see it using the index as expected if the query returns less than or about k rows. And of course, keep up to date with AskTOM via the official twitter account. Question and Answer. You Asked Team, Why don't oracle use index in this case, trace shows that Oracle don't even consider costing the index, just cost the Full table scan and picks that one as the final cost.

Well said, this seem to be one of the area, where optimizer has to be enhanced. Oracle Database compresses only nonpartitioned indexes that are nonunique or unique indexes of at least two columns. Restrictions on Online Indexes Online indexes are subject to the following restrictions:.

Parallel DML is not supported during online index building. For a nonunique secondary index on an index-organized table, the number of index key columns plus the number of primary key columns that are included in the logical rowid in the index-organized table cannot exceed The logical rowid excludes columns that are part of the index key.

This clause has been deprecated. Oracle Database now automatically collects statistics during index creation and rebuild. This clause is supported for backward compatibility and will not cause errors. This clause specifies the parameter string that is passed uninterpreted to the appropriate ODCI indextype routine. The maximum length of the parameter string is characters. If you are altering or rebuilding an entire index, then the string must refer to index-level parameters.

If you are rebuilding a partition of the index, then the string must refer to partition-level parameters. If you have installed Oracle Text, then you can rebuild your Oracle Text domain indexes using parameters specific to that product.

For more information on those parameters, please refer to Oracle Text Reference. Oracle Data Cartridge Developer's Guide for more information on indextype routines. ENABLE applies only to a function-based index that has been disabled because a user-defined function used by the index was dropped or replaced.

This clause enables such an index if these conditions are true:. The signature of the current function matches the signature of the function when the index was created. This clause lets you disable the use of a function-based index. You might want to do so, for example, while working on the body of the function. An unusable index must be rebuilt, or dropped and re-created, before it can be used.

You can execute statements that require the index if the statements do not access the unusable partition. You can also split or rename the unusable partition before rebuilding it. Restriction on Marking Indexes Unusable You cannot specify this clause for an index on a temporary table. Use this clause to rename an index.