Re: Optimizing bulk update performance

On 2013-04-27, Yang Zhang <yanghatespam@gmail.com> wrote:
> On Sat, Apr 27, 2013 at 1:55 AM, Misa Simic <misa.simic@gmail.com> wrote:

>> Optionaly you can run vacuum analyze after bulk operation...
>
> But wouldn't a bulk UPDATE touch many existing pages (say, 20%
> scattered around) to mark rows as dead (per MVCC)?  I guess it comes
> down to: will PG be smart enough to mark dead rows in largely
> sequential scans (rather than, say, jumping around in whatever order
> rows from foo are yielded by the above join)?

A plpgsql FOR-IN-query loop isn't going to be that smart, it's a
procedural language ans does things procedurally, if you want to do
set operations use SQL.

this:

 UPDATE existing-table SET .... FROM temp-table WHERE join-condition;

will likely get you a sequential scan over the existing table

and should be reasonably performant as long as temp-table is small
enough to fit in memory.

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