Thread: Let-bindings in SQL statements

Let-bindings in SQL statements

From
Jon Smark
Date:
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

Re: Let-bindings in SQL statements

From
Szymon Guz
Date:


On 26 January 2012 15:37, Jon Smark <jon.smark@yahoo.com> wrote:
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


In fact now() is a little bit tricky here. now() returns the time when the transaction started, so if you run `begin;` and call now() multiple times (even in different queries, but within the same transaction), the function will return the same value.

regards
Szymon

Re: Let-bindings in SQL statements

From
"David Johnston"
Date:
-----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.




Re: Let-bindings in SQL statements

From
David W Noon
Date:
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)
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*

Attachment

Re: Let-bindings in SQL statements

From
Filip Rembiałkowski
Date:


On Thu, Jan 26, 2012 at 3:37 PM, Jon Smark <jon.smark@yahoo.com> wrote:
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...


WITH param AS ( select now() as p_start, somefunc() as p_something )
SELECT * FROM param,my_table WHERE right_now >= param.p_start AND ...



Re: Let-bindings in SQL statements

From
Jasen Betts
Date:
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

Re: Let-bindings in SQL statements

From
Jasen Betts
Date:
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

Re: Let-bindings in SQL statements

From
Marti Raudsepp
Date:
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