Обсуждение: space usage of NULL fields

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space usage of NULL fields

От
Jan Vicherek
Дата:
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 :)
  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  ?
     Thanx,
        Jan
-- Gospel of Jesus is the saving power of God for all who believe --               ## To some, nothing is impossible.
##                 http://Vicherek.Waterloo.on.ca/

Re: [HACKERS] space usage of NULL fields

От
Bruce Momjian
Дата:
> 
>   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.


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