Обсуждение: About index ...

Hello,

First, excuse me for my english. I have 33 tables ( with millions rows ) and
for most tables i use btree index. My problem is when
i use explain directive. In fact btree index work only
with operator "=" . eg: select * from test where id = 45
EXPLAIN
Index scan using idx_id on test
an doesn't work with select * from test where id < 45.
EXPLAIN
Seq scan on test
Why ???

Thanks

<pejac@altern.org> writes:
> an doesn't work with select * from test where id < 45.
> EXPLAIN
> Seq scan on test

The system doesn't believe that WHERE clause is selective enough to be
efficiently done as an indexscan.  An indexscan will be slower than a
seqscan unless it is able to visit just a small fraction of the rows
in the table, and a one-sided inequality doesn't guarantee that.

If 45 happens to be near the bottom of the range of ID values in this
table, then indeed an indexscan might be a reasonable implementation.
But the planner won't know the range of values unless you've done a
VACUUM ANALYZE on this table.

            regards, tom lane