Craig James wrote:
Kevin Grittner wrote:
Which leaves the issue open -- a flexible way to flag the *reason* (or
*reasons*) for the absence of a value could be a nice enhancement, if
someone could invent a good implementation. Of course, one could
always add a column to indicate the reason for a NULL; and perhaps
that would be as good as any scheme to attach reason flags to NULL.
You'd just have to make sure the reason column was null capable for
those rows where there *was* a value, which would make the reason "not
applicable"....
I'd argue that this is just a special case of a broader problem of
metadata: Data about the data. For example, I could have a temperature,
40 degrees, and an error bounds, +/- 0.25 degrees. Nobody would think
twice about making these separate columns. I don't see how this is any
different from a person's middle initial of NULL, plus a separate column
indicating "not known" versus "doesn't have one" if that distinction is
important. There are many examples like this, where a simple value in
one column isn't sufficient, so another column contains metadata that
qualifies or clarifies the information. NULL is just one such case.
But, this should probably be on an SQL discussion board, not PG
performance...
Most DBMSs I'm aware of use a null *byte* attached to a nullable column
to indicate whether the column is null or not. yes/no takes one *bit*.
That leaves 255 other possible values to describe the state of the
column. That seems preferable to adding an additional column to every
nullable column.
But as you say, that would have to be taken up with the SQL
standardization bodies, and not PostgreSQL.
--
Guy Rouillier
--
Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
