Re: [HACKERS] space usage of NULL fields

>
> Hello,
>
> a quick question (I couldn't contact postgresql.org, so I didn't check
> the FAQ, but I doubt it would be there.)
>
> When I create a table with field "nnn float8" and total row size, say
> 140 bytes, and then I insert a row with NULL in this field, will my record
> take up those 8 bytes in physical page (say 140 bytes) or will it take up
> less (like 140-8=132), i.e. will it be shortened by those 8 bytes, since
> the value is NULL and no information is stored in that field ? ( I *hate*
> run-on sentences :)

Shorter. Nulls are not stored in the row. There is a bitmask on every
row that shows the number of NULL fields.

>
> In general, what are the savings/overheads associated in PG with
> NULL/not NULL fields ? I.e. if will I save the space if I define the field
> as varchar(8) instead of float8 ?

no. float8 is shorter because varchar has 4-byte overhead. The faq is
in docs/FAQ too.

--
Bruce Momjian | http://www.op.net/~candle
maillist(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026