Re: How to get started hacking on pgsql

Greg Stark kirjutas N, 04.12.2003 kell 19:55:
> I have an idea for what I think may be a very simple optimization for postgres
> to make. I would like to try my hand at implementing it, but the last time I
> tried I apparently started off in the wrong direction.
>
> In the following query, the sort step is completely unnecessary. The order is
> already guaranteed by the index:
>
>
> test=# create table test (a integer,b integer);
> CREATE TABLE
> test=# create index test_i on test(a,b);
> CREATE INDEX
> test=# explain select * from test where a=1 order by b;
> QUERY PLAN
> -------------------------------------------------------------------------
> Sort (cost=5.95..5.96 rows=6 width=8)
> Sort Key: b
> -> Index Scan using test_i on test (cost=0.00..5.87 rows=6 width=8)
> Index Cond: (a = 1)
> (4 rows)
>
> At what point in the process would it make sense to check for this?

Why not rewrite it as:

test=# explain select * from test where a=1 order by a,b;
---------------------------------------------------------------------
Index Scan using test_i on test (cost=0.00..17.07 rows=5 width=8)
Index Cond: (a = 1)
(2 rows)

> Where should I be looking in the code?

Try to find where the modified query is tested for. It's probably be
inside the optimizer, as index scan + no sort is not always faster than
seq scan + sort, as shown by the same query after vacuum analyze (on an
empty table)

hannu=# vacuum analyze test;
VACUUM
hannu=# explain select * from test where a=1 order by a,b;
QUERY PLAN
-----------------------------------------------------------
Sort (cost=0.01..0.02 rows=1 width=8)
Sort Key: a, b
-> Seq Scan on test (cost=0.00..0.00 rows=1 width=8)
Filter: (a = 1)
(4 rows)

---------------
Hannu