I need some basic advice on how to run a subquery, or if there's a
better way. Let me set up a situation, and get some advice on it. This
is my first post on this list, so I hope this kind of noob question is
ok.

Say I have a table of customers and table of accounts receivable
transactions, There is a one-to-many relationship between the two
(obviously a customer can have more than one purchase/transaction).

I want to run a query where I pull a set of customers based on some
parameter like, for sake of an example, where their status = new, and
also pull the most recent billing date from the accounts receivable
table.

Here's what I came up with:

SELECT customers.id, customers.firstname,
customers.lastname, customers.phone number,
(SELECT ar.billdate FROM ar
WHERE customers.customerid = ar.customerid
ORDER BY ar.billdate LIMIT 1)
AS lastarbilldate
FROM customers
WHERE customers.status = 'new';

As you can see, I'm using a subquery here to get the latest billing
date. My question is twofold. Is this subquery style the only way to
get one record in a one-to-many relationship, or is there a way to do it
with a join? Also, if this is fine, is there a way to do it that would
get the same results but be faster? It may not matter on a small
database, but if you've got millions of customers and transactions, a
subquery can get expensive.

I'm just looking for some basic direction. I hope my fake example makes
sense.

I'm running PostgreSQL 8.1, on a Windows 2003 server.

TIA

-Jeff
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try this approach:

SELECT
c.id,
c.firstname,
c.lastname,
a.latest_billdate
FROM
customers c
INNER JOIN -- or LEFT if you want the NULLs
(
SELECT
customer_id,
max(billdate) as latest_billdate
FROM
ar
) a
ON
c.customerid = a.customerid
WHERE
c.status = 'new';



> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailtogsql-general-owner@postgresql.org] On Behalf Of Demel, Jeff
> Sent: Thursday, February 01, 2007 8:08 AM
> To: pgsql-general@postgresql.org
> Subject: [GENERAL] Subqueries - performance and use question
>
> I need some basic advice on how to run a subquery, or if there's a
> better way. Let me set up a situation, and get some advice
> on it. This
> is my first post on this list, so I hope this kind of noob question is
> ok.
>
> Say I have a table of customers and table of accounts receivable
> transactions, There is a one-to-many relationship between the two
> (obviously a customer can have more than one purchase/transaction).
>
> I want to run a query where I pull a set of customers based on some
> parameter like, for sake of an example, where their status = new, and
> also pull the most recent billing date from the accounts receivable
> table.
>
> Here's what I came up with:
>
> SELECT customers.id, customers.firstname,
> customers.lastname, customers.phone number,
> (SELECT ar.billdate FROM ar
> WHERE customers.customerid = ar.customerid
> ORDER BY ar.billdate LIMIT 1)
> AS lastarbilldate
> FROM customers
> WHERE customers.status = 'new';
>
> As you can see, I'm using a subquery here to get the latest billing
> date. My question is twofold. Is this subquery style the only way to
> get one record in a one-to-many relationship, or is there a
> way to do it
> with a join? Also, if this is fine, is there a way to do it
> that would
> get the same results but be faster? It may not matter on a small
> database, but if you've got millions of customers and transactions, a
> subquery can get expensive.
>
> I'm just looking for some basic direction. I hope my fake
> example makes
> sense.
>
> I'm running PostgreSQL 8.1, on a Windows 2003 server.
>
> TIA
>
> -Jeff
> This email is intended only for the individual or entity to
> which it is addressed. This email may contain information
> that is privileged, confidential or otherwise protected from
> disclosure. Dissemination, distribution or copying of this
> e-mail or any attachments by anyone other than the intended
> recipient, or an employee or agent responsible for delivering
> the message to the intended recipient, is prohibited. If you
> are not the intended recipient of this message or the
> employee or agent responsible for delivery of this email to
> the intended recipient, please notify the sender by replying
> to this message and then delete it from your system. Any
> use, dissemination, distribution, or reproduction of this
> message by unintended recipients is strictly prohibited and
> may be unlawful.
>
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> broadcast)---------------------------
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>
> http://www.postgresql.org/docs/faq
>

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sorry, missing GROUP BY and some column naming was messed up but
hopefully you get the idea:

SELECT
c.id,
c.firstname,
c.lastname,
a.latest_billdate
FROM
customers c
INNER JOIN -- or LEFT if you want the NULLs
(
SELECT
customerid,
max(billdate) as latest_billdate
FROM
ar
GROUP BY
customerid
) a
USING
(customerid)
WHERE
c.status = 'new';


> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailtogsql-general-owner@postgresql.org] On Behalf Of George Pavlov
> Sent: Thursday, February 01, 2007 8:53 AM
> To: Demel, Jeff; pgsql-general@postgresql.org
> Subject: Re: [GENERAL] Subqueries - performance and use question
>
> try this approach:
>
> SELECT
> c.id,
> c.firstname,
> c.lastname,
> a.latest_billdate
> FROM
> customers c
> INNER JOIN -- or LEFT if you want the NULLs
> (
> SELECT
> customer_id,
> max(billdate) as latest_billdate
> FROM
> ar
> ) a
> ON
> c.customerid = a.customerid
> WHERE
> c.status = 'new';
>
>
>
> > -----Original Message-----
> > From: pgsql-general-owner@postgresql.org
> > [mailtogsql-general-owner@postgresql.org] On Behalf Of Demel, Jeff
> > Sent: Thursday, February 01, 2007 8:08 AM
> > To: pgsql-general@postgresql.org
> > Subject: [GENERAL] Subqueries - performance and use question
> >
> > I need some basic advice on how to run a subquery, or if there's a
> > better way. Let me set up a situation, and get some advice
> > on it. This
> > is my first post on this list, so I hope this kind of noob
> question is
> > ok.
> >
> > Say I have a table of customers and table of accounts receivable
> > transactions, There is a one-to-many relationship between the two
> > (obviously a customer can have more than one purchase/transaction).
> >
> > I want to run a query where I pull a set of customers based on some
> > parameter like, for sake of an example, where their status
> = new, and
> > also pull the most recent billing date from the accounts receivable
> > table.
> >
> > Here's what I came up with:
> >
> > SELECT customers.id, customers.firstname,
> > customers.lastname, customers.phone number,
> > (SELECT ar.billdate FROM ar
> > WHERE customers.customerid = ar.customerid
> > ORDER BY ar.billdate LIMIT 1)
> > AS lastarbilldate
> > FROM customers
> > WHERE customers.status = 'new';
> >
> > As you can see, I'm using a subquery here to get the latest billing
> > date. My question is twofold. Is this subquery style the
> only way to
> > get one record in a one-to-many relationship, or is there a
> > way to do it
> > with a join? Also, if this is fine, is there a way to do it
> > that would
> > get the same results but be faster? It may not matter on a small
> > database, but if you've got millions of customers and
> transactions, a
> > subquery can get expensive.
> >
> > I'm just looking for some basic direction. I hope my fake
> > example makes
> > sense.
> >
> > I'm running PostgreSQL 8.1, on a Windows 2003 server.
> >
> > TIA
> >
> > -Jeff
> > This email is intended only for the individual or entity to
> > which it is addressed. This email may contain information
> > that is privileged, confidential or otherwise protected from
> > disclosure. Dissemination, distribution or copying of this
> > e-mail or any attachments by anyone other than the intended
> > recipient, or an employee or agent responsible for delivering
> > the message to the intended recipient, is prohibited. If you
> > are not the intended recipient of this message or the
> > employee or agent responsible for delivery of this email to
> > the intended recipient, please notify the sender by replying
> > to this message and then delete it from your system. Any
> > use, dissemination, distribution, or reproduction of this
> > message by unintended recipients is strictly prohibited and
> > may be unlawful.
> >
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> >
> > http://www.postgresql.org/docs/faq
> >
>
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"Demel, Jeff" <Jeff.Demel@JavelinDirect.com> writes:
> Here's what I came up with:

> SELECT customers.id, customers.firstname,
> customers.lastname, customers.phone number,
> (SELECT ar.billdate FROM ar
> WHERE customers.customerid = ar.customerid
> ORDER BY ar.billdate LIMIT 1)
> AS lastarbilldate
> FROM customers
> WHERE customers.status = 'new';

Are you expecting a whole lot of answer rows from this query, or just a
few? If just a few, this way is fine, but if a lot you probably want to
try to recast it as a join. As is, it's pretty much like a nestloop
join of the two tables, which is good for a few result rows and not so
good for a lot.

The thing you need to make it fast as a subselect is an index on
(customerid, billdate) in that order. Oh, you also need PG 8.1 or
later, but I see you have that.

Personally I'd just write (SELECT max(billdate) FROM ar WHERE
customers.customerid = ar.customerid) rather than trying to get cute
with ORDER BY/LIMIT --- the planner versions that are able to handle
this case decently will deal with either one about as well.

regards, tom lane

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