Question

If I built a btree index on columns (one, two, three, four, five)
will it be used for a query with where block:

(1) ... WHERE one = 1 AND two = 2 AND three = 3 AND four = 4 -- no five

or

(2) ... WHERE two = 2 AND one = 1 -- wrong sequence, no five

or

(3) ... WHERE one = 1 AND three = 3 AND two = 2 AND five = 5 AND four = 4 -- wrong sequence

Answer 1

Yes in all your cases, because databases don’t generally worry about the order of statements in your WHERE clause.
However, it’s not likely to be used

WHERE two = 2 AND three = 3 -- no one

Because one is the first component of your index, but doesn’t appear in your where clause