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POMySql - slow sending data phase
 

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    <p>One of my queries on MySQL 5.0.45 is running slow in "sending data" phase. The query is a simple select, returns about 300 integer ID fields as result set.</p> <pre> mysql> SELECT source_id FROM directions WHERE (destination_id = 10); +-----------+ | source_id | +-----------+ | 2 | | 8 | ... | 2563 | +-----------+ 341 rows in set (2.13 sec) </pre> <p>I am note sure why is "sending data" phase so slow and what can be done to make it fast. Please note I am executing this query on MySQL prompt on the server itself, so not really expecting it to spend so much time in "sending data". Any clues?</p> <p>If it helps, I have 3 text fields on this table, but since they are not being selected, I am expecting they are not cause of this slowness.</p> <p>This query runs thousands of times a day and can't really afford to spend 2 secs on it each time.</p> <p>Profiling result:</p> <pre> mysql> show profile for query 4; +--------------------------------+----------+ | Status | Duration | +--------------------------------+----------+ | (initialization) | 0.000003 | | checking query cache for query | 0.000051 | | checking permissions | 0.000007 | | Opening tables | 0.000011 | | System lock | 0.000005 | | Table lock | 0.000023 | | init | 0.00002 | | optimizing | 0.00001 | | statistics | 0.00006 | | preparing | 0.000014 | | executing | 0.000005 | | Sending data | 2.127019 | | end | 0.000015 | | query end | 0.000004 | | storing result in query cache | 0.000039 | | freeing items | 0.000011 | | closing tables | 0.000007 | | logging slow query | 0.000047 | +--------------------------------+----------+ 18 rows in set (0.00 sec) </pre> <p>UPDATE: I stumbled upon the following URL which says</p> <pre> Each time means the time elapsed between the previous event and the new event. So, the line: | Sending data | 0.00016800 | means that 0.00016800 seconds elapsed between "executing" and "Sending data". It is, it takes 0.00016800 seconds to execute the query. </pre> <p><a href="http://forums.mysql.com/read.php?24,241461,242012#msg-242012" rel="noreferrer">http://forums.mysql.com/read.php?24,241461,242012#msg-242012</a></p> <p>Can somebody validate?</p>
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    1. COThe statement is not true on the linked page. If you do a `SHOW PROCESSLIST` while the query is running it will show that the query is indeed in the **Sending data** state. (Not in the previous one). - And please noone say that the processlist is showing the *next* step in the execution. :)
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