Re: Question about explain of index scan

Hannu Krosing <hannu(at)skype(dot)net> writes:
> If I get a plan like this, what will actually be performed if EXPLAIN
> shows this:

> Sort (cost=12.90..12.91 rows=1 width=207)
> Sort Key: log_actionseq
> -> Index Scan using sl_log_1_idx2_hu, sl_log_1_idx2_hu,
> sl_log_1_idx2_hu, sl_log_1_idx2_hu on sl_log_1 (cost=0.00..12.89 rows=1
> width=207)
> Index Cond: (
> ((log_xid < '1349053093') AND (log_xid >= '1349052761'))
> OR ((log_xid < '1349053093') AND (log_xid >= '1349052761'))
> OR ((log_xid < '1349053093') AND (log_xid >= '1349052761'))
> OR ((log_xid < '1349053093') AND (log_xid >= '1349052761'))
> )

> Will the same range be scanned 4 times ?

Yes. However, I don't understand how you got that result; AFAIK the
planner should have eliminated the duplicate subclauses. For example,
in 8.0 I get

regression=# explain select * from tenk1 where unique1 between 1 and 100 or unique1 between 1 and 100 or unique1 between 1 and 100;
QUERY PLAN
---------------------------------------------------------------------------------
Index Scan using tenk1_unique1 on tenk1 (cost=0.00..360.63 rows=102 width=244)
Index Cond: ((unique1 >= 1) AND (unique1 <= 100))
(2 rows)

Is Slony doing something to bypass the planner?

regards, tom lane