Hey, 2012/1/25 Merlin Moncure <mmonc...@gmail.com>
> On Wed, Jan 25, 2012 at 5:54 AM, Jason Armstrong <j...@riverdrums.com> > wrote: > > Hi > > > > I'm looking for advice on the best way to index a table that is defined > as: > > > > create table uuid.master(id uuid, parent uuid references > > uuid.master(id), type_id smallint, primary key(id)); > > > > Besides the primary key, I have these two indices on the table too: > > CREATE INDEX master_parent_idx ON uuid.master(parent); > > CREATE INDEX master_type_idx ON uuid.master(type_id); > > > > I have data arranged in four levels (ie type_id is from 1 to 4): > > > > 1. id=A type_id=1 > > 2. id=B parent=A type_id=2 > > 3. id=C parent=B type_id=3 > > 4. id=D parent=C type_id=4 > > 2. id=E parent=A type_id=2 > > 3. id=F parent=E type_id=3 > > 4. id=G parent=F type_id=4 > > 4. id=H parent=F type_id=4 > > 4. id=I parent=F type_id=4 > > 3. id=J parent=E type_id=3 > > 4. id=K parent=J type_id=4 > > > > I want to count all type_id=4 for a particular type_id=1 uuid. > > > > I use this query: > > > > SELECT count(t4.id) > > FROM uuid.master AS t4 > > INNER JOIN uuid.master AS t3 ON t4.parent=t3.id > > INNER JOIN uuid.master AS t2 ON t3.parent=t2.id > > INNER JOIN uuid.master AS t1 ON t2.parent=t1.id > > WHERE t1.id=UUID > > > > Apart from creating a separate table to keep track of the counts, is > > there a good way to index the table to help? > > If your data is organized as a tree, tends to run deep (causing many > recursion joins), and you often make sweeping subset operations > starting from a parent node, and your data isn't too heavily updated, > you might want to consider materialized path organization instead of > parent/child. Both arrays and strings work for that. > > id=I parents=A,E,F type_id=4 > > SELECT count(*) FROM uuid.master WHERE parents LIKE 'A,E%' and type_id = 4; > > The point here is that you can exploit the tree structure with a btree > index. Before we got recursive queries, this was often the best way > to do it, but now it's kind of a niche solution to be used when > certain things fall into place. > > merlin > Another approarch is to use ltree. It's easy and robust. http://www.postgresql.org/docs/9.1/static/ltree.html -- // Dmitriy.
