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Editorial Team
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Asked: 2026-06-14T22:06:49+00:00

I would like to know if the explain plan cost in Oracle always determine

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I would like to know if the explain plan cost in Oracle always determine if a specific query is most efficient (in terms of performance, resource usage, disk access, ect) than other?

My question is because I have two tables.

  • One with a local partition index.
  • The other with global partition index.

Both have the same structure and the same data. Then i have a query, the cost is significantly different. The global partition index one has a very small cost, and the local partition index one a very high cost.However when i run the queries in SQL Developer, the response time is higher for the table with global partition index.

Thanks.

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1 Answer

  1. Editorial Team

    Cost is not comparable across two different SQL statements. it cannot and should not be inferred that higher cost = higher runtime or IO/CPU usage.

    Cost is just an internal ranking that oracle applies when its calculating all possible sql plans for a specific sql statement.
    As you’ve seen, a low cost for one sql is longer to run than a high cost sql. the cost numbers are affected by a great many things such as sql hints (first_rows etc), table statistics, system level statistics (load stats, or setting different numbers on optimizer_index_cost_adj/optimizer_index_caching etc).

    Always tune SQL by IO/CPU (ie actual resource usage). ignore “cost” really.

    also see here: http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:313416745628

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