Re: Why is seq search preferred here by planner?

> Mallah,
>
>> tradein_clients=# begin work ; explain analyze UPDATE email_bank set
>> country=personal_account_details.country FROM personal_account_details where
> email_bank.userid > 0
>> and email_bank.userid=personal_account_details.userid and (
> email_bank.country <>
>> personal_account_details.country or email_bank.country IS NULL );BEGIN
>
> Ooops, yeah, you're correct ... the parens are required, I just forgot them.
>
> The reason that the planner is using a seq scan on personal_account_details is the same as the
> reason for using a seq scan on email_bank; the number of rows which match the condition,
> about 150,000. With that many qualifying rows, a seq scan is faster.

But there are two tables here , email_bank and personal_account_details
in personal account details only one row is supposed to match a given userid
as userid is the PKEY , why seq_scan there ? or am i getting the explain wrong ?

>
> How often do you do this query? If it's frequent, then running my version of the query with
> two new indexes -- one on email_bank.userid,
> email_bank.country, and one on personal_account_details.userid, country -- would be a
> interesting test with my version of the query. Though I'm not sure about the
> email_bank.country IS NULL condition; workarounds, anyone?

Hey this query is not even part of my production system,

I am doing a massive import of all the mess i have created since
last year ;-) into a strcity normal form with lots of RIs and checks.

>
> If you run this query only once a day, don't worry about it; run my version of the query, and
> it should finish in < 30 seconds, and that should be good enough, yes?

Not applicable!

>
> Oh, and a "Hash" is a "Hash Join" -- where the DB basically throws all of the rows from both
> tables in a big mess and picks out the ones that match.
:- )

Hmm.. although we have beaten around the bush but my question is still not answered
"why seq scan on a pkey " ?? (or am i getting the explain wrong )

anyway nevermind as my update is already over and i have added an
FKEY on the coutry feild to prevent collecting future mess.

Warm Regds
Mallah

>
> --
> -Josh Berkus
> Aglio Database Solutions
> San Francisco
>
>
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