Re: [PERFORM] A Better External Sort?

Ron,

On 9/30/05 1:20 PM, "Ron Peacetree" <rjpeace(at)earthlink(dot)net> wrote:

> That 11MBps was your =bulk load= speed. If just loading a table
> is this slow, then there are issues with basic physical IO, not just
> IO during sort operations.

Bulk loading speed is irrelevant here - that is dominated by parsing, which
we have covered copiously (har har) previously and have sped up by 500%,
which still makes Postgres < 1/2 the loading speed of MySQL.

> As I said, the obvious candidates are inefficient physical layout
> and/or flawed IO code.

Yes.

> Until the basic IO issues are addressed, we could replace the
> present sorting code with infinitely fast sorting code and we'd
> still be scrod performance wise.

Postgres' I/O path has many problems that must be micro-optimized away. Too
small of an operand size compared to disk caches, memory, etc etc are the
common problem. Another is lack of micro-parallelism (loops) with long
enough runs to let modern processors pipeline and superscale.

The net problem here is that a simple "select blah from blee order
by(blah.a);" runs at 1/100 of the sequential scan rate.

- Luke