So for every data page there is a 20 byte header, for every row there is a 4 byte identifier (offset into the page), AND there is also a 28 byte fixed-size header (27 + optional null bitmap)?? (I did find the section in the 8.1 manual that give the physical page layout.) The other RDBMS platforms I have worked with have a header in the 28 byte range and a row pointer of 4 bytes, and that's it. I find it a bit surprising that PostgreSQL would need another 28 bytes per row to track its contents.
I'll try the pg_relpages function as you suggest and recalculate from there. Thanks for the info, -Keaton On Thu, 2007-05-17 at 15:23 +0100, Heikki Linnakangas wrote: > Keaton Adams wrote: > > Using an 8K data page: > > > > 8K data page (8192 bytes) > > Less page header and row overhead leaves ~8000 bytes > > At 100 bytes per row = ~80 rows/page > > Rows loaded: 250,000 / 80 = 3125 data pages * 8192 = 25,600,000 bytes / > > 1048576 = ~ 24.4 MB of data page space. > > That's not accurate. There's 32 bytes of overhead per row, and that > gives you just 61 tuples per page. Anyhow, I'd suggest measuring the > real table size with pg_relpages function (from contrib/pgstattuple) or > from pg_class.relpages column (after ANALYZE). > > > We are running on PostgreSQL 8.1.4 and are planning to move to 8.3 when > > it becomes available. Are there space utilization/performance > > improvements in WAL logging in the upcoming release? > > One big change in 8.3 is that COPY on a table that's been created or > truncated in the same transaction doesn't need to write WAL at all, if > WAL archiving isn't enabled. >
