Hi,
I am asking because ...
I have a table with
relpages | 19164
reltuples | 194775
pg_relation_size / 8192 yields the same number as relpages. So, there is
no need to scale reltuples. Relcardinality is therefore 194775.
Statistics target is the default, 100. So, I assume each of the 100
buckets contains 1947.75 tuples.
Now, I have a timestamp column and a query for col>'2013-01-01'. There
are 27 buckets out of the 100 where col is >'2013-01-01'. The bucket
boundaries where 2013-01-01 falls into are
hist | 2013-01-08 20:48:52
hist | 2012-12-13 12:36:30
There is no / operation for INTERVAL types. So, I calculate in seconds:
select (27
+ extract('epoch' from
'2013-01-08 20:48:52'::timestamp
- '2013-01-01'::timestamp)
/ extract('epoch' from
'2013-01-08 20:48:52'::timestamp
- '2012-12-13 12:36:30'::timestamp))
* 1947.75;
That results in 53170.9642980797 and would be rounded to 53171.
However, EXPLAIN shows:
-> Seq Scan on client (cost=0.00..21731.03 rows=52939 width=29)
Filter: (date_joined > '2013-01-01 00:00:00'::timestamp
without time zone)
The numbers are of the same number of magnitude, but they are too
different to be rounding errors.
So, what did I wrong?
Thanks,
Torsten
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