Re: why sequential scan

Two estimates I undestand are quite
good.

select distinct id on body_index where string='book'

returns about 1500 rows.  That matches with
the bottom line of the plan

There are 5139 rows in table item.  It is
the same number of rows in the plan for
sequential scan

If I were doing a maual join I would do

q=> select distinct id on body_index where string='book'

which gives me an index scan

I would then iterate over each id I get and
look up in item like this

q=> select * from item where item =? order by finish

Explain gives me 1 row estimate for each lookup.
At most 1500 rows.  No?

Below is the original plan for easier reference
-------------
q=> explain select distinct h.id,i.item,i.heading,i.finish from item i ,body_index h where h.id=i.item and
+(h.string='book') order by finish;
NOTICE:  QUERY PLAN:

Unique  (cost=6591.46..6606.51 rows=150 width=24)
  ->  Sort  (cost=6591.46..6591.46 rows=1505 width=24)
        ->  Hash Join  (cost=5323.27..6512.04 rows=1505 width=24)
              ->  Seq Scan on item i  (cost=0.00..964.39 rows=5139 width=20)
              ->  Hash  (cost=5319.51..5319.51 rows=1505 width=4)
                    ->  Index Scan using body_index_string on body_index h  (cost=0.00..5319.51 rows=1505 width=4)
--------------


Thanks

On Thu, Aug 16, 2001 at 10:59:18AM -0400, Tom Lane wrote:
> newsreader@mediaone.net writes:
> > Can someone explain why pg is doing
> > a sequential scan on table item with the following
> > statement
>
> Looks like a fairly reasonable plan to me, if the rows estimates are
> accurate.  Are they?
>
>             regards, tom lane