Re: what is the difference between index (a,b) and (b,a)

[Benoit St-Jean]

There is no "one answer fits all" to this. Normally, the column that is the most selective should appear first. But, there are cases where you'd go the other way around to "reuse" the same index (and avoid creating another one). For instance :

Query 1 : SELECT * FROM tablename WHERE a=2 AND b=17
If column A is way more selective than column B, and index on (a, b) makes sense.

But let's say columns A and B more or less have the same selectivity and let's say you have a second query that is excuted often

Query 2: SELECT * FROM tablename WHERE b = 145.

You could add an index on B for this second query. So you'd have 2 indexes, (a,b) and (b). But indexes take place, they take time to update, etc. So in this case, perhaps you'd be better off with a single index that could be used by BOTH queries, namely an index on (b, a).