Order in Multi-Column Index Matters
Order of the columns matter.
The index is useless if the first column doesn't appear in the WHERE clause.
Example 1:
An index on (t, x) will not be used in this query:
- SELECT * FROM table WHERE x = 11
Example 2:
However an index on (x, t) will be used in both of these queries:
SELECT * FROM table WHERE x = 11
and
SELECT * FROM table WHERE t = 9 and x = 11
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