Svil,
Please advice me,
You have values and one table for N,R,P,F and O and Z, right?
And you have ownership which is a "catch-all" associative table between
values and whatever other table, is this correct?
You want to retrieve the values for a certain N, and all to all the other
entities that belong to that N, is this correct?
Fine,
I have a few questions on your associative tables
mm_N2P -- An N can be associated to P in a many to many relationship? N
Can have one or none P, am I right? It can have many Ps ? And a P can belong
to many Ns ?
mm_P2O, -- I understand an O can have many Positions, but a Positon can be
in more than one O? is this a many to many relationship?
mm_O2O, -- if it is a hierarchical tree do you need an associative table? An
O can have many sub-departments but a sub-department (O) can just belong to
a (super-)department,
-- isn't it so?
mm_N2Z -- this one was mistake of yours, it doesn't exist, right?
You wrote that giant query by yourself or was it generated by some automated
tool? (sqlalchemy , I have no idea :p )
Your ultimate goal is to retrieve the values associated with a certain
Department( and all its sub-departments) which is associated with a certain
Position, which values
are also to obtain, such position being associated with a certain N, which
values are also to obtain, is my understandin correct?
Explain me the simplest case, where you have an N not associated with any R
or P, just with an F.
Which info is to be retrieved, exactly in this case ?
Best,
Oliveiros
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <pgsql-sql@postgresql.org>
Sent: Tuesday, September 30, 2008 3:24 PM
Subject: Re: [SQL] optimizing a query over tree-like structure
another idea i just got, to decrease the number of tables in one FROM,
is to represent the first-level ORs with unions. but i'm not sure how
exactly to do it: are these equivalent?
FROM N,ownership,mm_N2R
where (
N.dbid = ownership.N
OR
N.dbid = mm_N2R.left AND mm_N2R.right = ownership.R
OR
...
) AND N.obj = whatever-filter-by-N
--------
FROM (
(ownership join N on ownership.N = N.dbid)
union
(ownership join mm_N2R on ownership.R = mm_N2R.right join N on
mm_N2R.left = N.dbid )
OR
) ...
and should i bundle the filtering-by-N/Employment in every of above
union-members?
On Tuesday 30 September 2008 15:21:09 Oliveiros Cristina wrote:
Hi, Svil
I 'd like to first fully understand the background of your problem
before figurin out if I can be of any help (or not).
You have a tree, of which N is the root, is this correct?
Then, what are the next sublevel?
F, P and R? If so, why is R linked to a sibling (F) ?
And the next one?
O and Z?
Is O connected to itself?
And i am not understanding your concept of "shortcuts". Could you
please explain ?
What kind of tree do you have exactly? Binary? Trenary?
The mm_* tables keep relations between nodes, I guess.... If so ,
the mm_N2Z one is empty, in this example, right?As there is no edge
from N to Z (not direct).
But what is the Nazn table? What records does it keep?
And what is the ownership table? And the value?
Could you tell which columns these tables have, at least the
relevant ones for your problem ?
Please kindly advice me on these issues.
I am not very specialized in optimizing queries, but I see you have
a lot of cartesian products on your FROM clause, which, from my own
experience, I guess it has tendency to be slow...
Best,
Oliveiros
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: <pgsql-sql@postgresql.org>
Sent: Tuesday, September 30, 2008 9:32 AM
Subject: [SQL] optimizing a query over tree-like structure
> hi.
> sorry for the vague syntax used below, but the query is huge so
> i've tried to present it in simple terms. And sorry if i'm doing
> obviously stupid things, i have lots of years programming behind
> me but NO sql involved.
> i have a somewhat tree-like structure of objects that link to
> each other via many2many associations. it looks like:
> (N is "root")
> N links to R,P,F
> R links to F
> P links to O,F
> O links to O,F #recursively
> F links to Z
> All links to F but the one in O are "shortcuts", to avoid looking
> it up recursively.
> each of these objects has some associated values (again
> many2many, ownership).
>
> what i want is to get all the values related to a given N and its
> sublevels, in one query.
>
> one variant of what i've invented so far is (~pseudocode, no
> recursion on O):
>
> SELECT ownership.*, value.*
> FROM Nazn, mm_N2P, mm_P2O, mm_O2O, mm_O2O AS mm_O2O1, mm_N2Z,
> ownership JOIN value ON ownership.value = value.dbid
> WHERE (
> N.dbid = ownership.N
> OR
> N.dbid = mm_N2R.left AND mm_N2R.right = ownership.R
> OR
> N.dbid = mm_N2P.left AND (
> mm_N2P.right = ownership.P
> OR
> mm_N2P.right = mm_P2O.left AND (
> mm_P2O.right = ownership.O
> OR
> mm_P2O.right = mm_O2O.left AND (
> mm_O2O.right = ownership.O
> OR
> mm_O2O.right = mm_O2O1.left AND
> mm_O2O1.right = ownership.O
> )))
> OR
> Nazn.dbid = mm_N2F.left AND (
> mm_N2F.right = ownership.F
> OR
> mm_N2Z.right = ownership.Z
> )
> ) AND ownership.value = value.dbid AND N.obj =
> whatever-filter-by-N
>
> ----------------
> this scales very poor.
> it uses the shortcut to F present in N.
> for just 200 rows with related associations, it takes 4 seconds
> to get result.
> if i use the shortcut to F present in P, it takes 2 seconds - but
> thats still inacceptable.
> seems that the number or consequtive ORs on same level is killing
> it. EXPLAIN gives nested loops all over.
> What am i doing wrong here?
> should i expand the A-to-B links of the sort
> mm_N2P.right = mm_P2O.left
> into
> mm_N2P.right = P.dbid and P.dbid == mm_P2O.left ?
>
> the query is generated via sqlalchemy and a layer on top, so i
> can tweak it any way required (and it has many other
> sub/filterings which i've ommited for brevity - they dont make it
> better/worse).
>
> any pointers of how such queries should be written are
> appreciated - e.g. what is considered fine, what doable and what
> is a no-no.
>
> thanks ahead
> ciao
> svil
>
> www.svilendobrev.com
> dbcook.sf.net
>
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