Francois Deliege wrote:

I am trying to estimate the size of a table composed of 51754000 rows.
Each row has 31 attributes: 16 x bit(24) and 15 x bit(8)

So, the payload should be:
51754000 x ( 16 x 24 + 15 x 24 ) bits = 3115 MB

What data types are those exactly? If those 24-bit fields are in fact text, varchar, char(x) or other data types that are stored as variable length fields, the varlen header will take 4 bytes. And then there's alignment, those 24-bit fields are most almost certainly 4-byte aligned, which means that there'll be one byte of padding between them.

The upcoming 8.3 release will be much better in that respect, It'll use just a 1 byte varlen header per field instead of 4 bytes for small values like yours. You might want to test a CVS snapshot.

Now, from what I understand from postgresql manual is that the overhead is composed of
32 bytes per row + 20 bytes per page.
This leads me to approx. 1700 MB overhead.
Therefore a total table size of 4900 MB.

In addition, there will be on average 1/2 rows worth of wasted space on every page.

  Heikki Linnakangas

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