Keaton Adams wrote:
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.
Yes, it is more than many other DBMSs. It contains mostly MVCC-related visibility information that other DBMSs store at page level etc, or don't have MVCC at all.
As Tom mentioned, that's going to be a bit better in 8.3. We reduced the header size from 27 + null bitmap to 23 + null bitmap, which makes a big difference especially on 64-bit architectures, where the header used to be padded up to 32 bytes, and now it's only 24 bytes.
For character fields, including CHAR(100) like you have, we also store a 4 bytes length header per field. That's been reduced to 1 byte for string shorter than 127 bytes in 8.3.
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