I am pretty new in optimizing tables with index and may need some help.
This is my query:
EXPLAIN SELECT timestamp
WHERE blockid = ‘200811252000’
ORDER BY timestamp DESC
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
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 !