hi
jus recently there were some thread on postgresql list with people asying : i have 700 columns, i have 1000 columns and so on.
some people, imediatelly responded: change your schema.
this is what forced to me ask:
i have a situation where i ahve to store a number of "objects" in database.
all objects have 3 specific attributes (which go into objects table), and may have a lot of "custom fields".
basically - lsit of accessible custom fields for object depends on which object-category this object belongs to.
now.
i know, i could have written it in this way:
create table object_custom_fields (id serial primary key, object_id int8, field_id int8, field_value text);
but:
this approach has two very big drawbacks (for me):
1. the table cannot differentiate between custom fields of type "date", "number" and so on. - everything is stored as text.
2. it is rather slow. i have to do a non-unique index scan over
object_custom_fields, get all records, and pivot it (on the client side
of curse) to make it usable.
i did it differently, definitelly not nicely, but i dont see any other
way to get this performance with unknown list of custom fields:
1. create table cf_types (id serial, codename text, representation text);
2. create table cf_definitions (id serial, category_id int8, type_id int8, field-number int4);
3. create table cf_values (id serial, object_id int8 (unique), ...................................................);
where
cf_types store information like this:
id | codename | representation
----+------------+----------------
1 | bool | boolean
2 | integer | integer
3 | number | number
4 | text | text
5 | note | text
6 | date | date
...
basically - there might be many "types" with the same representation.
then
cf_values have a lot of (128 at the moment) fields for all possible representations.
basically it looks like:
id, object_id, boolean_1 ... boolean_128, integer_1..integer_128, ...
the datatypes of this fields relate to their content (integer_* fields have datatype int8, and so on).
now.
in cf_definitions i specify, category, field_type_id, and a field-number - which relates to _NUMBER in fields in cf_values.
what i did achive is *very* fast retrieval of data for any given object.
the schema of cf_values table is absolutelly awful, and i will never say differently.
my point is - if somebody (tom lane for example) says - redesign your
schema - whenever he reads about table with 700 column (i have more :)
- then i must have missed something absolutelyl simple, fast and
elegant. what is this?
depesz
