a question about row estimation in postgres

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От Reynold Xin
Тема a question about row estimation in postgres
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Msg-id AANLkTimnoa4mS1SQN=gW1TYzRDwbbsghApMZLz4uMQUd@mail.gmail.com
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Ответы Re: a question about row estimation in postgres
Re: a question about row estimation in postgres
Список pgsql-general
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I have a rankings table and it has 1302 rows in total. I am a bit confused by how postgres (8.2.11) calculates 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;
                                   QUERY PLAN                                   
--------------------------------------------------------------------------------
 Index Scan using gradrate_idx on rankings  (cost=0.00..32.24 rows=7 width=196)
   Index Cond: (gradrate < 10::double precision)
(2 rows)



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

Shouldn't the row estimation be:

(11 - 8) / (33 - 8) / 10 * 1302 = 15.624
and
(10 - 8) / (33 - 8) / 10 * 1302 = 10.416

instead of 11 and 7?

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

--
Reynold Xin

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