A NULL costs almost nothing, in anycase, less than a real value.
There is the cost of the NULL bitmap, 1 bit per column for each column
if there are *any* NULLs in a row, but once you'ce got one, the rest
are free for that row.
This is in the documentation somewhere...
On Fri, Apr 02, 2004 at 12:18:47PM -0500, Randall Skelton wrote:
> What is the storage cost of a null entry in a column? i.e. does a null
> entry of type integer, float8 or text consume the same amount of
> storage as one that is filled? I ask because I have satellite data
> which is transmitted via a dodgy RF link that drops data packets. This
> means I have a number of columns in a table that are null. Moreover,
> the operations people decided to use a compression scheme whereby
> non-changing bit/integer values are not output at regular intervals
> which also adds a considerable number of null entries into the columns.
> Because of this, we made a decision that we would have hundreds of 2
> column tables (timestamp, value) and use unions, intersections, and
> joins to get what was needed. Unfortunately, this has made application
> programming a real nightmare as we are often forced to reconstruct a
> snapshot frame for the range of times either in C or have the app
> create temporary tables in SQL and insert the relevant data prior to
> selecting it. As it stands, we've ordered a new disk array and
> provided that the storage costs are not that high, I will probably be
> reorganising all this next week. If anyone has any other suggestions,
> I'd be very keen to hear them.
>
> Cheers,
> Randall
>
>
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Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
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