Depending on your problem domain, it might make sense to have multi-column
primary keys in some non-A tables, where a subset of their columns are the
foreign keys to the parents. In that case, you can skip some intermediary
tables in the joins. However, this would increase the size of your indexes and
slow updates.
A relevant question is whether these 5 tables all are self-similar or whether
each one is differently structured.
-- Darren Duncan
Menelaos PerdikeasSemantix wrote:
Let's say you have a father-child (or master-detail if you wish)
hierarchy of tables of not just 2 levels, but, say, 5 levels.
E.g. tables A, B, C, D and E organized in successive 1-to-N relationships:
A ----1-to-N-----> B
B ----1-to-N-----> C
C ----1-to-N-----> D
D ----1-to-N-----> E
with appropriate foreign keys:
* from E to D
* from D to C
* from C to B
* from B to A
This is normalized so far. Now assume that it is the case than in some
queries on table E you also need to report a field that only exists on
table A. This will mean a JOIN between five tables: E, D, C, B and A.
Some questions follow:
[1] assuming tables having a number of rows in the order of 100,000,
after how many levels of depth would you feel justified to depart from
the normalized schema and introduce some redundancy to speed up the queries?
[2] is adding redundant fields and extra foreign keys (say directly from
E to A) the best way to do this in 2012? Shouldn't some indexing and
fine tuning suffice ?
[3] do you feel this is a legitimate concern in a modern PostgreSQL
database running on high end (200,000 USD) hardware and serving no more
than 1000 concurrent users with table sizes at the lowest (more
detailed) level of the hierarchy in the order of a few tens of millions
of rows at the most and dropping by a factor of 20 for each level up ?
Menelaos.
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