Re: [HACKERS] Slow count(*) again...

On Thu, 3 Feb 2011, Robert Haas wrote:

> On Thu, Feb 3, 2011 at 3:54 PM,  <david@lang.hm> wrote:
>> with the current code, this is a completely separate process that knows
>> nothing about the load, so if you kick it off when you start the load, it
>> makes a pass over the table (competing for I/O), finishes, you continue to
>> update the table, so it makes another pass, etc. As you say, this is a bad
>> thing to do. I am saying to have an option that ties the two togeather,
>> essentially making the data feed into the Analyze run be a fork of the data
>> comeing out of the insert run going to disk. So the Analyze run doesn't do
>> any I/O and isn't going to complete until the insert is complete. At which
>> time it will have seen one copy of the entire table.
>
> Yeah, but you'll be passing the entire table through this separate
> process that may only need to see 1% of it or less on a large table.
> If you want to write the code and prove it's better than what we have
> now, or some other approach that someone else may implement in the
> meantime, hey, this is an open source project, and I like improvements
> as much as the next guy.  But my prediction for what it's worth is
> that the results will suck.  :-)

I will point out that 1% of a very large table can still be a lot of disk
I/O that is avoided (especially if it's random I/O that's avoided)

David Lang