SQL - 输入与非输入

Question

Suppose I have a table with column which takes values from 1 to 10. I need to select columns with all values except for 9 and 10. Will there be a difference (performance-wise) when I use this query:

SELECT * FROM tbl WHERE col NOT IN (9, 10)

and this one?

SELECT * FROM tbl WHERE col IN (1, 2, 3, 4, 5, 6, 7, 8)

Answer 1

Use "IN" as it will most likely make the DBMS use an index on the corresponding column.

"NOT IN" could in theory also be translated into an index usage, but in a more complicated way which DBMS might not "spend overhead time" using.

Answer 2

When it comes to performance you should always profile your code (ie run your queries few thousand times and measure each loops performance using some kind of stopwatch . Sample ).

But here I highly recommend using the first query for better future maintaining. The logic is that you need all records but 9 and 10. If you add value 11 to your table and use second query, logic of your application will be broken that will lead to bug, of course.

Edit: I remember this was tagged as php that's why I provided sample in php, but I might be mistaken. I guess it won't be hard to rewrite that sample in the language you're using.

Answer 3

I have seen Oracle have trouble optimizing some queries with NOT IN if columns are nullable. If you can write your query either way, IN is preferred as far as I'm concerned.

Answer 4

For a list of constants, MySQL will internally expand your code to:

SELECT * FROM tbl WHERE ((col <> 9 and col <> 10))

Same for the other one, with 8 times = instead.

So yes, the first one will be faster, less comparisons to be done. Chances that it is measurable are negligible though, the overhead of a handful of constant comparisons is nothing compared to the general overhead of parsing SQL and retrieving data.

Answer 5

"IN" statement works internally like a serie of "OR" statements.

For example:

SELECT * FROM tbl WHERE col IN (1, 2, 3)

Its equals to

SELECT * FROM tbl WHERE col = 1 OR col = 2 OR col = 3

"OR" statements could cause some performance issues as explained in this article: https://bertwagner.com/2018/02/20/or-vs-union-all-is-one-better-for-performance/

When you do a NOT IN statement, its all the same, but the result has a logical denial. BUT, you could write and equivalent query much better in performance. In your example:

SELECT * FROM tbl WHERE col NOT IN (9, 10)

Its equals to

SELECT * FROM tbl WHERE col <> 9 AND col <> 10

With an "AND" statement, the database stop analizing when one of all conditionals its false, so, its much better in performance than "OR" used in "IN" statement.