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POunderstanding explain plan in oracle
 

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  1. POunderstanding explain plan in oracle
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    <p>I was trying to understand the explain plan in oracle and wanted to know what conditions oracle considers while forming the explain plan</p> <p>I was testing a simple query in HR schema present in oracle 11g</p> <pre><code>select * from countries where region_id in (select region_id from regions where region_name = 'Europe'); </code></pre> <p>When I ran the following queries:</p> <pre><code>explain plan for select * from countries where region_id in (select region_id from regions where region_name = 'Europe'); SELECT * FROM table(dbms_xplan.display(null,null,'basic')); </code></pre> <p>I got the following output in the explain table:</p> <pre><code>-------------------------------------------------------- | Id | Operation | Name | -------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | NESTED LOOPS | | | 2 | INDEX FULL SCAN | COUNTRY_C_ID_PK | | 3 | TABLE ACCESS BY INDEX ROWID| REGIONS | | 4 | INDEX UNIQUE SCAN | REG_ID_PK | -------------------------------------------------------- </code></pre> <p>Here I observed that the outer query was executed first, i.e countries table was executed first as indicated by Row 3.</p> <p>Now I added an index on the region_name of the regions table and ran the explain plan again and got the following output</p> <pre><code>-------------------------------------------------------------- | Id | Operation | Name | -------------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | NESTED LOOPS | | | 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | | 3 | INDEX RANGE SCAN | REGIONNAME_REGIONS_IX | | 4 | INDEX UNIQUE SCAN | COUNTRY_C_ID_PK | | 5 | INDEX RANGE SCAN | COUNTRIES_REGIONID_IX | -------------------------------------------------------------- </code></pre> <p>Now my question is:</p> <ol> <li>Shouldn't the inner query be executed first irrespective of whether index is present or not</li> <li>If the adding an index alters the execution plan, what other features can alter it?</li> <li>In general case what is the execution process like is it sequential (first executes the join which occurs first and then goes to next join in the query) ?</li> </ol> <p>Thanks in advance for your help.</p> <p>-Varun</p>
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