in reply to Re^3: How to interpolate sql-output
in thread How to interpolate sql-output

Let's say that I have table ppl, which includes running id, activity boolean and timestamp defaulting to now.
Then I have another table (=ppl_data) where is also id as serial (only for easier manipulation), ppl_id, key and value and usually timestamp.

I can put anything to that ppl_data-table without altering db-schema, cause it is in key-value-pair.

Cause I use only inserts and fetching only newest data with that key, there is nice history of changes.

Reason to this is, that customer is able to add different fields if needed.
Ofc there is also datatype for that field, so sanity-check and casting is easier.

If needed one can vacuum old data from that ppl_data, if it affects to db-server, but usually there is enough power.
As log as that db-server isn't virtual-server where disk-latency might cause slowing downs eventually.

And when there is also way to do different queries, one can alter or even add features without code-changes.

Replies are listed 'Best First'.
Re^5: How to interpolate sql-output
by chacham (Prior) on Jan 15, 2015 at 00:25 UTC

    Oh, i think we have a misunderstanding. I wasn't saying the two table structure is bad. I meant that the query requires only one table.

    The structure you are using is known as Entity Attribute Value. It is excellent when working with unknown data, though some efforts may be required to keep it working efficiently.

[reply]

      Ok. Now I see your point more clearly.

      Proper indexing is essential in that key-value-case.

      Actually, that 'parent'-table is needed, cause it contains main activity boolean.
      I just forgot to add that and activity after that variable.

      I had no idea of name of that approach.
      It's just natural derivation after associative arrays and key-values in Memcache etc.

      My fools dream is to put everything to db, even code.
      Just a Apache module and conf to Apache, how to find code.
      I think that it could make updates and backuping easier.
      Ofc that code should be cached, but then servers could be even lighter. Cause it can work without hard drive (or ssd).

      If one, like me, wants to balanced and redundant web-serving it would be great if servers are light and easily swappable.
      Also there are quite significant power savings, when one can shut down some servers in quieter times.

      One can save a lot compared normal one unit server in both power and cooling costs.

[reply]
[d/l]

        I just forgot to add that and activity after that variable.

        Details, details, details... :)

        The query might be better as a join:

        select distinct
    foo.id        key,
    last_value(data.value)
     over
     (
      partition by
        data.foo_id
      order by
        data.id
     )            value
from
    foo,
    foo_data    data
where
        foo.id        <> ?
  and    foo.activity    is true
        —
  and    data.foo_id    = foo.id
  and    data.aktivity    is true
  and    data.key        = 'name';

        [download]

        It's just natural derivation after associative arrays and key-value

        That is great for programming. In the database world, it is generally considered the wrong approach, when the data is known. If data captured is unknown, it is likely the only choice.

        Proper indexing is essential in that key-value-case.

        That will work on small tables. Once it gets large, the index is of little help. Partitioned tables is one way out. In any case, EAVs are best avoided.

        put everything to db, even code.

        People do that, but it is error prone and confusing. Besides, queries can be stored natively in the database as a view or inside a stored procedure. That is what they are there for.

[reply]
[d/l]

In Section Seekers of Perl Wisdom