On Mon, 6 Jun 2005, Tom Lane wrote:
Junji TERAMOTO <[EMAIL PROTECTED]> writes:
In "whole buffer log", there is a page header that includes offset of
"hole" (lower and upper). If we use that information, we don't need
any overhead, do we?
No, because the WAL code cannot assume that all pages follow the
convention that pd_lower and pd_upper represent the boundaries of
free space. (As a counterexample: index metapages don't always
do that.) I think the transformation has to be guaranteed lossless,
which means that at a minimum you'd need to check whether the data
in between pd_lower and pd_upper really is zeroes. So the irreducible
minimum overhead is 1 bit to tell whether you compressed or not.
Vacuum doesn't zero out the free space between lower and upper, it's
just marked as unused, so a lossless compression becomes less efficient
on tables that have free space released by vacuum in them.
How about adding a flag to XLogRecData to indicate if the space between
pd_lower and pd_upper is meaningful or not? The XLogInsert caller probably
knows that. That way you could completely skip over the free space if
it's not meaningful, saving even more cycles.
- Heikki
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