On Fri, Apr 27, 2012 at 1:51 AM, Josh Berkus wrote:

Now, the other issue I'd be worried about for this optimization is what
happens when the nulls become non-trailing?  For example, this pattern:

1. Out of 700 columns, columns 301+ are all Null, so we map them away.
2. User updates column 688 to non-null
3. Suddenly we have a MUCH larger row which will no longer fit on the page.

If your application had a lot of that kind of update pattern, I'd be
concerned that this would be a deoptimzation.
Currently, we have a long row before and a long row after. Jamie's
proposals would give us a short row before and a long row after.

Since we don't ever update in place, we're much more likely to fit on
the same page with this optimisation than without it. I guess we can
check that with a performance test.

(Perhaps a more obvious optimisation would be to use a compressed NULL
bitmap. That would respond better in a wider range of use cases than
just truncation of trailing zeroes.)

--
Simon Riggs                   http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

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