RE: [HACKERS] Caution: tonight's commits force initdb

Hi

> -----Original Message-----
> From: owner-pgsql-hackers(at)postgreSQL(dot)org
> [mailto:owner-pgsql-hackers(at)postgreSQL(dot)org]On Behalf Of Tom Lane
> Sent: Saturday, August 21, 1999 12:58 PM
> To: pgsql-hackers(at)postgreSQL(dot)org
> Subject: [HACKERS] Caution: tonight's commits force initdb
>
>
> I have just committed changes that alter the representation of
> SortClause nodes (making them like GroupClause, instead of the
> crufty way they were done before). This breaks stored rules!
> You will need to initdb next time you pull current sources...
>
> The good news is that the optimizer is finally reasonably smart
> about avoiding a top-level Sort operation.
>

Thanks for your good jobs.

After applying this change,I tested some cases.
For a table t,explain shows

explain select * from t;

NOTICE: QUERY PLAN:

Seq Scan on t (cost=1716.32 rows=27131 width=612)

And with ORDER BY clause

explain select * from t order by key;

NOTICE: QUERY PLAN:

Index Scan using t_pkey on t (cost=2284.55 rows=27131 width=612)

Hmm,Index scan is chosen to select all rows.
AFAIK,sequential scan + sort is much faster than index scan in
most cases.

cost of index scan < cost of sequential scan + cost of sort

I have felt that the current cost estimation of index scan is too small,
though I have no alternative.

Comments ?

Hiroshi Inoue
Inoue(at)tpf(dot)co(dot)jp