Question

Suppose I have two queries on a database table.

The queries are defined in terms of fields used in the query:

Query1: depends on f1, f2, and f3

Query2: depends on f1, f2, f3 and f4

I remember reading somewhere that the SQL query engine (mySQL in this case) parses the index tree starting from the leftmost fields in the index.

If that is correct, then I assume that instead of having two indices defined on the table like this:

Index 1 (for Query1) : CREATE INDEX idx_1 {f1, f2, f3}
Index 2 (for Query2) : CREATE INDEX idx_2 {f1, f2, f3, f4}

I can simply define one index which contains the union of the keys used in both queries – i.e.

I only need to define this index:

(for BOTH Query1) : CREATE INDEX the_idx {f1, f2, f3, f4}

I have two questions:

1. Is my assumption correct?. i.e. can I simply define the one index (the_idx) instead of the previous two?

2. Does this index behavior hold true for PostgreSQL query engine as well?

Answer 1

Is my assumption correct?. i.e. can I simply define the one index (the_idx) instead of the previous two?

Yes.
It’s called a covering index, and you want to order the columns based on which is most likely to be used queries. IE: If f2 is the most common column, you’d want to use:

CREATE INDEX the_idx {f2, f1, f3, f4}

Does this index behavior hold true for PostgreSQL query engine as well?

No, Postgres does not support covering indexes.

Indexes are not ANSI standard; it’s a miracle the terminology is so consistent between vendors as it is.