nickf nickf - 1 year ago 222
MySQL Question


Out of the following queries, which method would you consider the better one? What are your reasons (code efficiency, better maintainability, less WTFery)...

SELECT MIN(`field`)
FROM `tbl`;

SELECT `field`
FROM `tbl`
ORDER BY `field`

Answer Source

In the worst case, where you're looking at an unindexed field, using MIN() requires a single full pass of the table. Using SORT and LIMIT requires a filesort. If run against a large table, there would likely be a significant difference in percieved performance. As a meaningless data point, MIN() took .36s while SORT and LIMIT took .84s against a 106,000 row table on my dev server.

If, however, you're looking at an indexed column, the difference is harder to notice (meaningless data point is 0.00s in both cases). Looking at the output of explain, however, it looks like MIN() is able to simply pluck the smallest value from the index ('Select tables optimized away' and 'NULL' rows) whereas the SORT and LIMIT still needs needs to do an ordered traversal of the index (106,000 rows). The actual performance impact is probably negligible.

It looks like MIN() is the way to go - it's faster in the worst case, indistinguishable in the best case, is standard SQL and most clearly expresses the value you're trying to get. The only case where it seems that using SORT and LIMIT would be desirable would be, as mson mentioned, where you're writing a general operation that finds the top or bottom N values from arbitrary columns and it's not worth writing out the special-case operation.

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