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POOptimizing a mysql index on an ordered, range query
 

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  1. POOptimizing a mysql index on an ordered, range query
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    <p>I'm looking for help with some offensive mysql queries I currently have running against my server. My goal is to show the most expensive ebay items with end time's less than one month ago.</p> <p>I'm using MySQL 5.1.</p> <p>My query is as follows ('ebay_items' has ~350,000 rows):</p> <pre><code>explain SELECT `ebay_items`.* FROM `ebay_items` WHERE (endtime &gt; NOW()-INTERVAL 1 MONTH) ORDER BY price desc\G; </code></pre> <p>yields:</p> <pre><code>*************************** 1. row *************************** id: 1 select_type: SIMPLE table: ebay_items type: range possible_keys: endtime key: endtime key_len: 9 ref: NULL rows: 71760 Extra: Using where; Using filesort 1 row in set (0.00 sec) </code></pre> <p>This query results in an expensive 'filesort' using 71760 rows. </p> <pre><code>show indexes on ebay_items; </code></pre> <p>yields (I've only included the index in question, 'endtime'):</p> <pre><code>*************************** 7. row *************************** Table: ebay_items Non_unique: 1 Key_name: endtime Seq_in_index: 1 Column_name: endtime Collation: A Cardinality: 230697 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: *************************** 8. row *************************** Table: ebay_items Non_unique: 1 Key_name: endtime Seq_in_index: 2 Column_name: price Collation: A Cardinality: 230697 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: </code></pre> <p>Only the 'endtime' key of the composite endtime index (endtime, price) is being used. As far as I know, MySQL will not make use of a composite index effectively when dealing with a range query in conjunction with an 'order by' clause. </p> <p>Has anyone found a good workout for this issues? I'd primarily like to solve it at the DB level (either by a smarter use of indexes or schema changes) but I'm open to suggestions. </p> <p>One way that I could avoid the range query is to have a background task cycling through every X hours and marking an enum type field on ebay_items as '&lt; 1 day old', '&lt; 1 week old', '&lt; 1 month old', etc. I was hoping to solve the problem in a cleaner way.</p> <p>Is there any way to execute MySQL range query with an order by clause, queries in an efficient manner? </p> <p>Huge thanks for your help! </p> <p>Edit: Kohányi Róbert made the good point that I should clarify the exact problem I was having with the query. The query results in the disk I/O being pegged for it's duration. If several of these queries are running simultaneously, processes get backed up and the machine locks up. My assumption is that the filesort is eating the I/O. </p> <p>I should also mention that the table is using the MyISAM engine. Would it more performant and less I/O intensive to use the InnoDB engine? Thanks again.</p>
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    1. USWill Sulzer
    1. USWill Sulzer
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    1. COdo you have index on the `price` column only?
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      1. USRiyono
    2. COHello Riyono. I do not have an index on the `price` column explicitly. Do you believe that this would help?
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      1. USWill Sulzer
    3. COAt first i think it would help, but Kohányi Róbert spell it nicely on how MySQL behave when we have different keys for `WHERE` and `GROUP BY`. Do you only need the specific range of "less than a month ago"? If yes, i suggest you create different table to keep the ones that fall into this category and another for the rest. Perhaps `ebay_items` for the active month, and periodically move items which pass a month to `ebay_items_history` or something similar.
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      1. USRiyono
 

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