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POIndexes, EXPLAIN PLAN, and record access in Oracle SQL
 

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  1. POIndexes, EXPLAIN PLAN, and record access in Oracle SQL
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    <p>I have been learning about indexes in Oracle SQL, and I wanted to conduct a small experiment with a test table to see how indexes really worked. As I discovered from an earlier post made here, the best way to do this is with EXPLAIN PLAN. However, I am running into something which confuses me.</p> <p>My sample table contains attributes (EmpID, Fname, Lname, Occupation, .... etc). I populated it with 500,000 records using a java program I wrote (random names, occupations, etc). Now, here are some sample queries with and without indexes:</p> <p>NO INDEX:</p> <pre><code>SELECT Fname FROM EMPLOYEE WHERE Occupation = 'DOCTOR'; </code></pre> <p>EXPLAIN PLAN says:</p> <pre><code>OPERATION OPTIMIZER COST TABLE ACCESS(FULL) TEST.EMPLOYEE ANALYZED 1169 </code></pre> <p>Now I create index:</p> <pre><code>CREATE INDEX occupation_idx ON EMPLOYEE (Occupation); </code></pre> <p>WITH INDEX "occupation_idx":</p> <pre><code>SELECT Fname FROM EMPLOYEE WHERE Occupation = 'DOCTOR'; </code></pre> <p>EXPLAIN PLAN says:</p> <pre><code>OPERATION OPTIMIZER COST TABLE ACCESS(FULL) TEST.EMPLOYEE ANALYZED 1169 </code></pre> <p>So... the cost is STILL the same, 1169? Now I try this:</p> <p>WITH INDEX "occupation_idx":</p> <pre><code>SELECT Occupation FROM EMPLOYEE WHERE Occupation = 'DOCTOR'; </code></pre> <p>EXPLAIN PLAN says:</p> <pre><code>OPERATION OPTIMIZER COST INDEX(RANGE SCAN) TEST.OCCUPATION_IDX ANALYZED 67 </code></pre> <p>So, it appears that the index only is utilized when that column is the only one I'm pulling values from. But I thought that the point of an index was to unlock the entire record using the indexed column as the key? The search above is a pretty pointless one... it searches for values which you already know. The only worthwhile query I can think of which ONLY involves an indexed column's value (and not the rest of the record) would be an aggregate such as COUNT or something.</p> <p>What am I missing?</p>
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    1. COInteresting question, I would be curious to see why this is happening.
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    2. COIs a primary key defined for the `EMPLOYEE` table?
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    3. COIs it possible the index hadn't finished building? if you re-run the query SELECT Fname FROM EMPLOYEE WHERE Occupation = 'DOCTOR'; is its cost dropped?
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      1. USxQbert
 

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