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POSelecting both MIN and MAX From the Table is slower than expected
 

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  1. POSelecting both MIN and MAX From the Table is slower than expected
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    <p>I have a table <code>MYTABLE</code> with a date column <code>SDATE</code> which is the primary key of the table and has a unique index on it. </p> <p>When I run this query:</p> <pre><code>SELECT MIN(SDATE) FROM MYTABLE </code></pre> <p>it gives answer instantly. The same happens for: </p> <pre><code>SELECT MAX(SDATE) FROM MYTABLE </code></pre> <p>But, if I query both together:</p> <pre><code>SELECT MIN(SDATE), MAX(SDATE) FROM MYTABLE </code></pre> <p>it takes much more time to execute. I analyzed the plans and found when one of min or max is queried, it uses INDEX FULL SCAN(MIN/MAX) but when both are queried at the same time, it does a FULL TABLE SCAN. </p> <p>why?</p> <p><strong>Test Data:</strong></p> <p>version <code>11g</code></p> <pre><code>create table MYTABLE ( SDATE DATE not null, CELL VARCHAR2(10), data NUMBER ) tablespace CHIPS pctfree 10 pctused 40 initrans 1 maxtrans 255 storage ( initial 64K minextents 1 maxextents unlimited ); alter table MYTABLE add constraint PK_SDATE primary key (SDATE) using index tablespace SYSTEM pctfree 10 initrans 2 maxtrans 255 storage ( initial 64K minextents 1 maxextents unlimited ); </code></pre> <p>Load table:</p> <pre><code>declare i integer; begin for i in 0 .. 100000 loop insert into MYTABLE(sdate, cell, data) values(sysdate - i/24, 'T' || i, i); commit; end loop; end; </code></pre> <p>Gather stats: </p> <pre><code>begin dbms_stats.gather_table_stats(tabname =&gt; 'MYTABLE', ownname =&gt; 'SYS'); end; </code></pre> <p>Plan1:</p> <p><img src="https://i.stack.imgur.com/jHwT8.jpg" alt="enter image description here"></p> <p>Plan2:</p> <p><img src="https://i.stack.imgur.com/MVgsh.jpg" alt="enter image description here"></p>
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    1. This table or related slice is empty.
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      1. POSelecting both MIN and MAX From the Table is slower than expected
      1. This table or related slice is empty.
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      1. POSelecting both MIN and MAX From the Table is slower than expected
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      1. POSelecting both MIN and MAX From the Table is slower than expected
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    1. COHow many rows in the table? How fresh are the statistics?
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    2. COMy table has almost 100000 rows and stats are fresh; you can easily re-generate the problem by creating a simple table with only one or two columns and see the results for yourself.
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    3. COwhat are the costs for the queries? can you post the plans? I think the index is very fragmented.
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      1. POSelecting both MIN and MAX From the Table is slower than expected
      1. USFlorin Ghita
 

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