Subject Re: [firebird-support] How much space does a NULL take?
Author Ann W. Harrison
aiylam_s wrote:
> I have 2 Firebird databases, each with 10,000 records.
> The second has 4 VARCHAR columns:
> - ID VARCHAR(40) indexed not-null
> - LOCATION VARCHAR(250) indexed not-null
> The size of the first database on disk is 1941504 bytes (~1.9 MB).
> The size of the second on disk is 14700544 bytes (~14.7MB). Do 20,000
> NULLS really take up 12759040 bytes (~12.7MB)? That corresponds to
> about 638 bytes per NULL!
> Can anybody shed some light on this?

Yup. The null is only one bit, but the record is laid out with all
fields fully expanded and null fields zeroed or blanked. Then the
record is compressed, using a one byte run-length encoding. Up to 127
identical bytes turn into two bytes, which is OK, but not great when you
have 32,000 byte fields - that's a bit more than 500 bytes... and I've
probably forgotten something.