Thread: The "many nulls" problem

Kynn,

have you seen contrib/hstore ? You can have one table with common attributes
and hide others in hstore

Oleg
On Fri, 14 Mar 2008, Kynn Jones wrote:

> It often happens that a particular pieces of information is non-null for a
> small minority of cases.  A superficially different manifestation of this is
> when two pieces of information are identical in all but a small minority of
> cases.  This can be easily mapped to the previous description by defining a
> null in one column to mean that its contents should be obtained from those
> of another column.  A further variant of this is when one piece of
> information is a simple function of another one in all but a small minority
> of cases.
>
> (BTW, I vaguely recall that RDb theorists have a technical term for this
> particular design issue, but I don't remember it.)
>
> In all these cases, the design choice, at least according to RDb's 101, is
> between including a column in the table that will be NULL most of the time,
> or defining a second auxiliary column that references the first one and
> holds the non-redundant information for the minority of cases for which this
> is necessary (and maybe define a VIEW that includes all the columns).
>
> But for me it is a frequent occurrence that my quaint and simple RDb's 101
> reasoning doesn't really apply for PostgreSQL.  Basically, Pg is too smart
> for it!  For example, does a large proportion of NULLs really imply a lot of
> wasted space?  Maybe this is true for fixed-length data types, but what
> about for type TEXT or VARCHAR?
>
> Just to be concrete, consider the case of a customers database for some home
> shopping website.  Suppose that, as it happens, for the majority of this
> site's customers, the shipping and billing addresses are identical.  Or
> consider the scenario of a company in which, for most employees, the email
> address can be readily computed from the first and last name using the rule
> First M. Last => first_last@acme.com, but the company allows some
> flexibility for special cases (e.g. for people like Yasuhiro Tanaka who's
> known to everyone by his nickname, Yaz, the email is
> yaz_tanaka@acme.combecause hardly anyone remembers or even knows his
> full name.)
>
> What's your schema design approach for such situations?  How would you go
> about deciding whether the number of exceptional cases is small enough to
> warrant a second table?  Of course, one could do a systematic profiling of
> various possible scenarios, but as a first approximation what's your
> rule-of-thumb?
>
> TIA!
>
> Kynn
>

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83

Kynn Jones wrote:
> In all these cases, the design choice, at least according to RDb's 101, is
> between including a column in the table that will be NULL most of the time,
> or defining a second auxiliary column that references the first one and
> holds the non-redundant information for the minority of cases for which this
> is necessary (and maybe define a VIEW that includes all the columns).
>
> But for me it is a frequent occurrence that my quaint and simple RDb's 101
> reasoning doesn't really apply for PostgreSQL.  Basically, Pg is too smart
> for it!  For example, does a large proportion of NULLs really imply a lot of
> wasted space?

It depends. If there's *any* NULLs on a row, a bitmap of the NULLs is
stored in the tuple header. Without NULL bitmap, the tuple header is 23
bytes, and due to memory alignment, it's always rounded up to 24 bytes.
That one padding byte is "free" for use as NULL bitmap, so it happens
that if your table has eight columns or less, NULLs will take no space
at all. If you have more columns than that, if there's *any* NULLs on a
row you'll waste a whole 4 or 8 bytes (or more if you have a very wide
table and go beyond the next 4/8 byte boundary), depending on whether
you're on a 32-bit or 64-bit platform, regardless of how many NULLs
there is.

That's on 8.3. 8.2 and earlier versions are similar, but the tuple
header used to be 27 bytes instead of 23, so you have either one or five
"free" bytes, depending on architecture.

In any case, that's pretty good compared to many other RDBMSs.

 > Maybe this is true for fixed-length data types, but what
 > about for type TEXT or VARCHAR?

Datatype doesn't make any difference. Neither does fixed vs variable length.

> What's your schema design approach for such situations?  How would you go
> about deciding whether the number of exceptional cases is small enough to
> warrant a second table?  Of course, one could do a systematic profiling of
> various possible scenarios, but as a first approximation what's your
> rule-of-thumb?

 From performance point of view, I would go with a single table with
NULL fields on PostgreSQL.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

On Fri, 14 Mar 2008, Kynn Jones wrote:

> On Fri, Mar 14, 2008 at 2:59 PM, Oleg Bartunov <oleg@sai.msu.su> wrote:
>
>> have you seen contrib/hstore ? You can have one table with common
>> attributes
>> and hide others in hstore
>>
>
> That's interesting.  I'll check it out.  Thanks!

actually, hstore was designed specially for this kind of problems.


>
> Kynn
>

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83