Re: Why is seq search preferred here by planner?

> Mallah,
>
>> > 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 ?
>
> It doesn't matter whether it's a primary key or not. If your query or subquery condition
> returns a large number/proportion of rows, a seq scan is going to be faster than an index
> scan.
>
> As far as userid being a key, this affects the planner's *join method*, *not* how the database
> gets the rows.

Got it , you mean to say userid being pkey is affecting the method of
implicit join being made by the update stmt.

thanks once again.

Regds
mallah.

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

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