Let-bindings in SQL statements

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

-----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 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 ...

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