Thread: 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..50.47 rows=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..270.73 rows=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..20442.55 rows=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

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

In short, the planner estimates that "tradedate BETWEEN '20070801' and
'20070901'" matches more rows than "tradatedate = '20070801'"

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com