Обсуждение: Let-bindings in SQL statements

Hi,

Is it possible to do the equivalent of let-bindings in a pure SQL function?
I have a SELECT that invokes "now" multiple times.  It would be nicer
to do it only once and reuse the value.  Something like this:

LET right_now = SELECT now () IN
SELECT * FROM my_table WHERE right_now >= start AND ...

In PL/pgSQL this is easy, but I wonder about SQL...

Thanks in advance!
Jon

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Jon Smark
Sent: Thursday, January 26, 2012 9:38 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Let-bindings in SQL statements

Hi,

Is it possible to do the equivalent of let-bindings in a pure SQL function?
I have a SELECT that invokes "now" multiple times.  It would be nicer to do
it only once and reuse the value.  Something like this:

LET right_now = SELECT now () IN
SELECT * FROM my_table WHERE right_now >= start AND ...

In PL/pgSQL this is easy, but I wonder about SQL...

Thanks in advance!
Jon

-----------------------------------------------------------------------

No, not really.  In some cases you can use a CTE (WITH) clause to create a
single row with whatever names and values you need and then, using
Sub-Selects or CROSS JOIN, introduce that row into the appropriate parts of
the query.

 David J.

On Thu, 26 Jan 2012 06:37:49 -0800 (PST), Jon Smark wrote about
[GENERAL] Let-bindings in SQL statements:

>Is it possible to do the equivalent of let-bindings in a pure SQL
>function? I have a SELECT that invokes "now" multiple times.  It would
>be nicer to do it only once and reuse the value.  Something like this:
>
>LET right_now = SELECT now () IN
>SELECT * FROM my_table WHERE right_now >= start AND ...
>
>In PL/pgSQL this is easy, but I wonder about SQL...

Try using CURRENT_TIMESTAMP instead.  In fact, CURRENT_TIMESTAMP is
more traditional SQL than now().  I don't have an ANSI standard handy,
so I cannot be certain when now() was added, if ever; but I have been
using CURRENT TIMESTAMP (space instead of vinculum) under DB2 for 20
years or more.
--
Regards,

Dave  [RLU #314465]
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
dwnoon@ntlworld.com (David W Noon)
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*

On 2012-01-26, David Johnston <polobo@yahoo.com> wrote:

> Is it possible to do the equivalent of let-bindings in a pure SQL function?
> I have a SELECT that invokes "now" multiple times.  It would be nicer to do
> it only once and reuse the value.  Something like this:

There is no need. now() is tagged as stable. it will only be executed once.

the planner will figure this out for you.

--
⚂⚃ 100% natural

On 2012-01-26, David W Noon <dwnoon@ntlworld.com> wrote:
>
> Try using CURRENT_TIMESTAMP instead.  In fact, CURRENT_TIMESTAMP is
> more traditional SQL than now().  I don't have an ANSI standard handy,
> so I cannot be certain when now() was added, if ever; but I have been
> using CURRENT TIMESTAMP (space instead of vinculum) under DB2 for 20
> years or more.

The planner will rewrite CURRENT_TIMESTAMP to now()

 :)

--
⚂⚃ 100% natural

On Sat, Feb 11, 2012 at 12:42, Jasen Betts <jasen@xnet.co.nz> wrote:
> There is no need. now() is tagged as stable. it will only be executed once.
> the planner will figure this out for you.

Actually that's not always true. In index condition arguments, the
expression would indeed be executed just once. But in filter clauses
(e.g. seq scan), the whole expression is executed once per row, which
is a bit inefficient. Of course this makes no visible difference for
now(), since it always returns the same value -- the transaction start
time.

(I submitted a patch to improve this, but it's not certain whether it
will be included in PostgreSQL 9.2 or not)

Regards,
Marti