Problem re Set Like Structures

Hi,

Sorry if this is a duplicate post - I posted
yesterday, but I'm pretty sure from the wrong account,
and the Email hasn't shown on the list.  Also there
was an error in the original Email!

Whilst I'm new to Postgresql, I have long experience
in database work in general.  I am currently beginning
work on a project for which Postgresql is the
preferred database component.  The environment is
basically RedHat 7.x, Postgresql 7.0.2 and Python 2.x.

The problem relates to performing Set like operations
on data, specifically determining whether Set A is a
subset of Set B.  I've read through the Momjian book
and am just beginning on the docs, but can't find a
data structure to support Set like operations.  For a
while I became enthused by the possibilities of
arrays, but alas I was to be dissapointed.  This query
type occurs VERY frequently in the application and I
can see no efficient solution.

The problem is as follows:

We have three tables TABLE_A, TABLE_B and TABLE_C.
TABLE_C represents an attribute that records in the
other two tables may have references to.  The
relationship between tables A and B on the one hand,
and table C on the other are many-to-many.  Given no
group like structure, the only way I can see to
represent these relationships is by Join tables
joining TABLE_A<=>TABLE_C and TABLE_B<=>TABLE_C.

For any record 'a' in TABLE_A there will be a set
A_JOIN_C(a)={c1,c2,...,cn} of records in TABLE_C which
represent the keys of TABLE_C for which joins exist to
record 'a'.  Similarly, for each record {b1,b2,...,bn}
in TABLE_B there will exist a corresponding set
B_JOIN_C(bi).

The problem is for a given record 'a' in TABLE_A to
find the set of records in TABLE_B for which
B_JOIN_C(bi) is a subset of A_JOIN_C(a).

Now, I'm not saying that I can't formulate the query
sequence - that's easy.  The problem is in finding a
tolerably efficient solution without a native GROUP
data type.  The issue is that this query type occurs
so frequently in the problem domain that an efficient
solution is mandatory.  Does anyone have any suggestions?

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