a question about row estimation in postgres

[Reynold Xin <rxin@xxxxxxxxxxxxxxx>]

I have a rankings table and it has 1302 rows in total. I am a bit confused by how postgres (8.2.11)Âcalculates the cardinality for this rankings table based on < predicates on gradrate attribute.

select histogram_bounds from pg_stats where attname = 'gradrate' and tablename = 'rankings';
          histogram_bounds
------------------------------------
 {8,33,40,46,55,61,69,75,81,90,118}

explain SELECT * FROM rankings WHERE gradrate < 11;
                                   QUERY PLAN                                   
---------------------------------------------------------------------------------
 Index Scan using gradrate_idx on rankings  (cost=0.00..44.24 rows=11 width=196)
   Index Cond: (gradrate < 11::double precision)
(2 rows)

explain select * from rankings where gradrate < 10;

                 ÂQUERY PLAN                 ÂÂ

--------------------------------------------------------------------------------

ÂIndex Scan using gradrate_idx on rankings Â(cost=0.00..32.24Ârows=7Âwidth=196)

 ÂIndex Cond: (gradrate < 10::double precision)

(2 rows)

Following the formula outlined inÂhttp://www.postgresql.org/docs/8.3/static/row-estimation-examples.html

Both gradrate 10 and gradrate 11 would fall in the first bucket.

Shouldn't the row estimation be:

(11 - 8) / (33 - 8) / 10 * 1302 =Â15.624

and

(10 - 8) / (33 - 8) / 10 * 1302 =Â10.416

instead of 11 and 7?

Perhaps I am missing something. I'd appreciate if you can point it out. Thanks!

--

Reynold Xin