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MySQL can perform the same optimization on col_name IS NULL that it can use for col_name = constant_value. For example, MySQL can use indexes and ranges to search for NULL with IS NULL . Share
POP1 is a float column that can also be NULL. POP1 IS NOT NULL should exclude about 50% of the entries, that's why I put it at the beginning. All other terms reduce the number only marginally. Amongst others, I designed an index pop1_vt_source, which seems to be not used, while an index with vt as first column is used.
7 lut 2012 · Anyway, while it is true that you cannot index a column containing a null value, there are ways to do it like for example: create index MY_INDEX on emp(ename, 1); notice the , 1) at the end which does the trick.
Yes, a prefix length of one will allow you to have a working index for looking up NULLs. Here's the SQLFiddle: http://sqlfiddle.com/#!2/12bf3/1/0 The EXPLAIN says that it is using the desired index.
5.3.4.6 Working with NULL Values. The NULL value can be surprising until you get used to it. Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values. To test for NULL, use the IS NULL and IS. NOT NULL operators, as shown here:
Columns can be NULL unless the index is a primary key. The index type for a non-SPATIAL index depends on the storage engine. Currently, B-tree is used. Permitted for a column that can have NULL values only for InnoDB, MyISAM, and MEMORY tables.
MySQL UNIQUE Index & NULL. Unlike other database systems, MySQL considers NULL values as distinct values. Therefore, you can have multiple NULL values in the UNIQUE index. This is how MySQL was designed. It is not a bug even though it was reported as a bug.
