How much disk-space is needed to store a NULL value using postgresql DB?
SqlPostgresqlTypesNullableSql Problem Overview
let's say I have a column on my table defined the following:
"MyColumn" smallint NULL
Storing a value like 0, 1 or something else should need 2 bytes (1). But how much space is needed if I set "MyColumn" to NULL? Will it need 0 bytes?
Are there some additional needed bytes for administration purpose or such things for every column/row?
(1) http://www.postgresql.org/docs/9.0/interactive/datatype-numeric.html
Sql Solutions
Solution 1 - Sql
Laramie is right about the bitmap and he links to the right place in the manual. Yet, this is almost, but not quite correct:
> So for any given row with one or more nulls, the size added to it > would be that of the bitmap(N bits for an N-column table, rounded up).
One has to factor in data alignment. The HeapTupleHeader (per row) is 23 bytes long, actual column data always starts at a multiple of MAXALIGN (typically 8 bytes). That leaves one byte of padding that can be utilized by the null bitmap. In effect NULL storage is absolutely free for tables up to 8 columns.
After that, another MAXALIGN (typically 8) bytes are allocated for the next MAXALIGN * 8(typically 64) columns. Etc. Always for the total number of user columns (all or nothing). But only if there is at least one actual NULL value in the row.
I ran extensive tests to verify all of that. More details:
Solution 2 - Sql
Null columns are not stored. The row has a bitmap at the start and one bit per column that indicates which ones are null or non-null. The bitmap could be omitted if all columns are non-null in a row. So for any given row with one or more nulls, the size added to it would be that of the bitmap(N bits for an N-column table, rounded up).
More in depth discussion from the docs here
Solution 3 - Sql
It should need 1 byte (0x00) however it's the structure of the table that makes up most of the space, adding this one value might change something (Like adding a row) which needs more space than the sum of the data in it.
Edit: Laramie seems to know more about null than me :)