MySql - slow sending data phase

Question

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.

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Answer 1

An explain-plan is usually the best place to start whenever you have a slow query. To get one, run

DESCRIBE SELECT source_id FROM directions WHERE (destination_id = 10);

This will show you a table listing the steps required to execute your query. If you see a large value in the 'rows' column and NULL in the 'key' column, that indicates that your query having to scan a large number of rows to determine which ones to return.

In that case, adding an index on destination_id should dramatically speed your query, at some cost to insert and delete speed (since the index will also need to be updated).