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POAdding an Index degraded execution time
 

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  1. POAdding an Index degraded execution time
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    <p>I have a table like this:</p> <pre><code>myTable (id, group_id, run_date, table2_id, description) </code></pre> <p>I also have a index like this:</p> <pre><code>index myTable_grp_i on myTable (group_id) </code></pre> <p>I used to run a query like this:</p> <pre><code>select * from myTable t where t.group_id=3 and t.run_date='20120512'; </code></pre> <p>and it worked fine and everyone was happy. Until I added another index:</p> <pre><code>index myTable_tab2_i on myTable (table2_id) </code></pre> <p>My life became miserable... it's taking almost as 5 times longer to run !!! execution plan looks the same (with or without the new index):</p> <pre><code>-------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 220 | 17019 |* 1 | TABLE ACCESS BY INDEX ROWID| MYTABLE | 1 | 220 | 17019 |* 2 | INDEX RANGE SCAN | MYTABLE_GRP_I | 17056 | | 61 -------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("T"."RUN_DATE"='20120512') 2 - access("T"."GROUP_ID"=3) </code></pre> <p>I have almost no hair left on my head, why should another index which is not used, on a column which is not in the where clause make a difference ... </p> <hr> <p>I will update the things I checked:<br> a. I removed the new index and it run faster<br> b. I added the new index in 2 more different environments and the same thing happen<br> c. I changed MYTABLE_GRP_I to be on columns run_date and group_id - this made it run fast as a lightning !!</p> <p>But still why does it happen ?</p>
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    1. PTQuestion
    1. USAPC
    1. USBassal
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    1. COI don't think it has to do anything with your problem, but is `RUN_DATE` of type `varchar2` ?? if so, this is a bad practice ...
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      1. USA.B.Cade
    2. COAre the table and index statistics up-to-date? Can you just drop the second index? Is it needed for other queries?
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      1. USSzilard Barany
    3. CO@A.B.Cade- yes, thanks for your comment
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