Difference in query plan when using = or > in where clause

Hi,

I am curious as to why this occurs. Why does an = change the query plan so
drastically?

When my query is:

Select count(*) from View_A WHERE tradedate = '20070801';
The query plan is as below: I see that the scan on the alloctbl is being
indexed on k_alloctbl_blockid_status

-> Bitmap Index Scan on idx_tradeblocktbl_tradeate
(cost=0.00..50.47rows=1444 width=0) (actual time=
0.040..0.040 rows=106 loops=1)
Index Cond: ((tradedate >=
'2007-08-01'::date) AND (tradedate <= '2007-09-24'::date))
-> Bitmap Heap Scan on alloctbl a
(cost=4.59..270.73rows=70 width=16) (actual time=
0.010..0.011 rows=1 loops=7)
Recheck Cond: (tr.recid = a.blockid)
-> Bitmap Index Scan on k_alloctbl_blockid_status (cost=
0.00..4.59 rows=70 width=0) (actual time=0.007..0.007 rows=1 loops=7)
Index Cond: (tr.recid = a.blockid)
Total runtime: 1.453 ms

But when my query is:
Select count(*) from View_A WHERE tradedate BETWEEN '20070801' and
'20070901';
The query plan is:

-
-> Bitmap Heap Scan on tradeblocktbl tr (cost=
50.47..2849.67 rows=1444 width=80) (actual time=0.095..0.218 rows=104
loops=1)
Recheck Cond: ((tradedate >= '2007-08-01'::date)
AND (tradedate <= '2007-09-24'::date))
-> Bitmap Index Scan on
idx_tradeblocktbl_tradeate (cost=0.00..50.47 rows=1444 width=0) (actual
time=0.050..0.050 rows=106 loops=1)
Index Cond: ((tradedate >=
'2007-08-01'::date) AND (tradedate <= '2007-09-24'::date))
-> Sort (cost=99007.79..100479.68 rows=588755 width=16)
(actual time=2660.009..3150.887 rows=588755 loops=1)
Sort Key: a.blockid
-> Seq Scan on alloctbl a
(cost=0.00..20442.55rows=588755 width=16) (actual time=
0.026..764.833 rows=588755 loops=1)

Total runtime: 3590.715 ms

Thank you.
Radhika

--
It is all a matter of perspective. You choose your view by choosing where to
stand. --Larry Wall