Sandman Sandman - 1 month ago 6
MySQL Question

Retrieving only a fixed number of rows in MySQL

I am testing my database design under load and I need to retrieve only a fixed number of rows (5000)

I can specify a LIMIT to achieve this, however it seems that the query builds the result set of all rows that match and then returns only the number of rows specified in the limit. Is that how it is implemented?

Is there a for MySQL to read one row, read another one and basically stop when it retrieves the 5000th matching row?

Answer

MySQL is smart in that if you specify a LIMIT 5000 in your query, and it is possible to produce that result without generating the whole result set first, then it will not build the whole result.

For instance, the following query:

SELECT * FROM table ORDER BY column LIMIT 5000

This query will need to scan the whole table unless there is an index on column, in which case it does the smart thing and uses the index to find the rows with the smallest column.