| Avoided filesort, but more rows to exmaine, need advice! [message #3743] |
Wed, 26 November 2008 16:54 |
 matthew016
Messages: 4
Registered: November 2008
Location: Brussels |
Junior Member |
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Hi,
I am pretty new in optimizing tables with index and may need some help.
This is my query:
EXPLAIN SELECT timestamp
FROM Meting_INT_COPY
WHERE blockid = '200811252000'
ORDER BY timestamp DESC
LIMIT 1
If I have an index(blockid),
EXPLAIN will return the following information:
type possible_keys key rows Extra
ref index_blockid index_blockid 2638 Using where; Using filesort
If I add an index(blockid,timestamp)
EXPLAIN will display the following:
type possible_keys key
rows Extra
ref index_blockid,index_blockid_timestamp index_blockid_timestamp 8248
Using where; Using index
The index(blockid,timestamp) avoid the filesort + returns the result from index ! (Using where; Using index)
But why for the index(blockid) 2638 rows are returned to examine and for a more
specific index(blockid,timestamp) 8248 rows are returned ?
Thank you very much for any answer !
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