Re: hstore - Implementation and performance issues around its operators

Hi Robert

Many thanks for your answers.

2011/7/19 Robert Haas <robertmhaas@gmail.com>:
> Putting the elements in order wouldn't really help, would it?  I mean,
> you'd need some kind of an index inside the hstore... which there
> isn't.

Sorry for my inprecise question. In fact elements of a hstore are
stored in order of (keylength,key) with the key comparison done
bytewise (not locale-dependent). See e.g. function hstoreUniquePairs
in http://doxygen.postgresql.org/ . This ordered property is being
used by some hstore functions but not all - and I'm still wondering
why.

Yours, Stefan


2011/7/19 Robert Haas <robertmhaas@gmail.com>:
> On Sun, Jun 19, 2011 at 2:59 PM, Stefan Keller <sfkeller@gmail.com> wrote:
>> 1. Obviously the '@>' has to be used in order to let use the GiST index.
>> Why is the '->' operator not supported by GiST ('->' is actually
>> mentioned in all examples of the doc.)?
>
> Because it's not a comparison operator.
>
>> 2. Currently the hstore elements are stored in order as they are
>> coming from the insert statement / constructor.
>> Why are the elements not ordered i.e. why is the hstore not cached in
>> all hstore functions (like hstore_fetchval etc.)?
>
> Putting the elements in order wouldn't really help, would it?  I mean,
> you'd need some kind of an index inside the hstore... which there
> isn't.
>
>> 3. In the source code 'hstore_io.c' one finds the following enigmatic
>> note: "... very large hstore values can't be output. this could be
>> fixed, but many other data types probably have the same issue."
>> What is the max. length of a hstore (i.e. the max. length of the sum
>> of all elements in text representation)?
>
> I think that anything of half a gigabyte or more is at risk of falling
> down there.  But probably it's not smart to use such big hstores
> anyway.
>
>> 4. Last, I don't fully understand the following note in the hstore
>> doc. (http://www.postgresql.org/docs/current/interactive/hstore.html
>> ):
>>> Notice that the old names are reversed from the convention
>>> formerly followed by the core geometric data types!
>>
>> Why names? Why not rather 'operators' or 'functions'?
>
> It's referring to the operator names.
>
>> What does this "reversed from the convention" mean concretely?
>
> That comment could be a little more clear, but I think what it's
> saying is that hstore's old @ is like the core geometic types old ~,
> and visca versa.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>