"Adam Pritchard" <vesper76(at)gmail(dot)com> writes:
> CREATE TABLE t ( c1 INT, c2 INT );
> CREATE INDEX c1_idx ON t(c2);
> CREATE INDEX c2_idx ON t(c2);
> EXPLAIN SELECT * FROM t ORDER BY c1, c2;
> "Sort (cost=69.83..72.33 rows=1000 width=8)"
> " Sort Key: c1, c2"
> " -> Seq Scan on t (cost=0.00..20.00 rows=1000 width=8)"
> If we instead use an index on c1, c2:
> CREATE INDEX c1_c2_idx ON t(c1, c2);
> "Index Scan using c1_c2_idx on t (cost=0.00..52.00 rows=1000 width=8)"
The latter index matches the sort ordering requested by the query;
no single-column index can do so.
regards, tom lane