Thread: PL/pgsql EXECUTE 'SELECT INTO ...'
I have looked a little bit at what it'd take to make SELECT INTO inside
an EXECUTE work the same as it does in plain plpgsql --- that is, the
INTO should reference plpgsql variables, not a destination table.
It looks to me like this is possible but would require some nontrivial
re-engineering inside plpgsql. What I'm visualizing is that EXECUTE
should read its string argument not just as an SPI_exec() string, but
as an arbitrary plpgsql proc_stmt. This would offer some interesting
capabilities, like building a whole FOR-loop for dynamic execution.
But there are a number of problems to be surmounted, notably arranging
for the parsetree built by the plpgsql compiler not to be irretrievably
memory-leaked. (That ties into something I'd wanted to do anyway,
which is to have the plpgsql compiler build its trees in a memory
context associated with the function, not via malloc().)
This does not look like something to be tackling when we're already
in late beta, unfortunately. So we have to decide what to do for 7.1.
If we do nothing now, and then implement this feature in 7.2, we will
have a backwards compatibility problem: EXECUTE 'SELECT INTO ...'
will completely change in meaning.
I am inclined to keep our options open by forbidding EXECUTE 'SELECT
INTO ...' for now. That's more than a tad annoying, because that leaves
no useful way to do a dynamically-built SELECT, but if we don't forbid
it I think we'll regret it later.
Comments?
regards, tom lane
Tom, > I have looked a little bit at what it'd take to make > SELECT INTO inside > an EXECUTE work the same as it does in plain plpgsql --- > that is, the > INTO should reference plpgsql variables, not a > destination table. > It looks to me like this is possible but would require > some nontrivial > re-engineering inside plpgsql. What I'm visualizing is <snip> > (That ties into something I'd wanted to > do anyway, > which is to have the plpgsql compiler build its trees in > a memory > context associated with the function, not via malloc().) All of this sounds good, but as a *heavy* PL/pgSQL user, it's still going off on somewhat of a tangent. As far as I'm concerned, the EXECUTE method was just a workaround for the lack "object" variables. What I always would rather have had is simply being able to drop a variable ... or an OID ... into a SELECT statement and not bothering with EXECUTE at all. > This does not look like something to be tackling when > we're already > in late beta, unfortunately. I'd agree with that. :-) > I am inclined to keep our options open by forbidding > EXECUTE 'SELECT > INTO ...' for now. That's more than a tad annoying, > because that leaves > no useful way to do a dynamically-built SELECT, but if we > don't forbid > it I think we'll regret it later. Unfortunately, I have already used EXECUTE in several functions ... my search routines will be hard to run without it. Perhaps you could turn off EXECUTE by default, but allow it as a compile-time option for those of us wise enough to understand the dangers? -Josh Berkus ______AGLIO DATABASE SOLUTIONS___________________________ Complete information technology josh@agliodbs.com and datamanagement solutions (415) 565-7293 for law firms, small businesses fax 621-2533 and non-profit organizations. San Francisco
Tom Lane wrote:
> I have looked a little bit at what it'd take to make SELECT INTO inside
> an EXECUTE work the same as it does in plain plpgsql --- that is, the
> INTO should reference plpgsql variables, not a destination table.
> It looks to me like this is possible but would require some nontrivial
> re-engineering inside plpgsql. What I'm visualizing is that EXECUTE
> should read its string argument not just as an SPI_exec() string, but
> as an arbitrary plpgsql proc_stmt. This would offer some interesting
> capabilities, like building a whole FOR-loop for dynamic execution.
> But there are a number of problems to be surmounted, notably arranging
> for the parsetree built by the plpgsql compiler not to be irretrievably
> memory-leaked. (That ties into something I'd wanted to do anyway,
> which is to have the plpgsql compiler build its trees in a memory
> context associated with the function, not via malloc().)
>
> This does not look like something to be tackling when we're already
> in late beta, unfortunately. So we have to decide what to do for 7.1.
> If we do nothing now, and then implement this feature in 7.2, we will
> have a backwards compatibility problem: EXECUTE 'SELECT INTO ...'
> will completely change in meaning.
>
> I am inclined to keep our options open by forbidding EXECUTE 'SELECT
> INTO ...' for now. That's more than a tad annoying, because that leaves
> no useful way to do a dynamically-built SELECT, but if we don't forbid
> it I think we'll regret it later.
You can do something like
FOR record_var IN EXECUTE <string-expr> LOOP ... END LOOP;
In this case, the <string-expr> executed over SPI_exec() must return tuples (0-n). Otherwise you'll get a runtime
error.
Inside the loop you have access to the tuples via the record. Is that the dynamically-built SELECT capability
you'vebeen missing?
There's not that much need for mucking with temp tables in EXECUTE as all this discussion looks to me.
Jan
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<p><font size="2">What I wrote wasn't about temp tables, it was about selecting into plpgsql variables. It would appearthat Jan's syntax gets around this problem.</font><p><font size="2">MikeA</font><br /><p><font size="2">-----OriginalMessage-----</font><br /><font size="2">From: Jan Wieck [<a href="mailto:janwieck@Yahoo.com">mailto:janwieck@Yahoo.com</a>]</font><br/><font size="2">Sent: 08 February 2001 13:30</font><br/><font size="2">To: Tom Lane</font><br /><font size="2">Cc: Jan Wieck; pgsql-hackers@postgreSQL.org; pgsql-sql@postgreSQL.org</font><br/><font size="2">Subject: [SQL] Re: PL/pgsql EXECUTE 'SELECT INTO ...'</font><br /><p><fontsize="2">Tom Lane wrote:</font><br /><font size="2">> I have looked a little bit at what it'd take to make SELECTINTO inside</font><br /><font size="2">> an EXECUTE work the same as it does in plain plpgsql --- that is, the</font><br/><font size="2">> INTO should reference plpgsql variables, not a destination table.</font><br /><font size="2">>It looks to me like this is possible but would require some nontrivial</font><br /><font size="2">> re-engineeringinside plpgsql. What I'm visualizing is that EXECUTE</font><br /><font size="2">> should read its stringargument not just as an SPI_exec() string, but</font><br /><font size="2">> as an arbitrary plpgsql proc_stmt. This would offer some interesting</font><br /><font size="2">> capabilities, like building a whole FOR-loopfor dynamic execution.</font><br /><font size="2">> But there are a number of problems to be surmounted, notablyarranging</font><br /><font size="2">> for the parsetree built by the plpgsql compiler not to be irretrievably</font><br/><font size="2">> memory-leaked. (That ties into something I'd wanted to do anyway,</font><br/><font size="2">> which is to have the plpgsql compiler build its trees in a memory</font><br /><fontsize="2">> context associated with the function, not via malloc().)</font><br /><font size="2">></font><br /><fontsize="2">> This does not look like something to be tackling when we're already</font><br /><font size="2">>in late beta, unfortunately. So we have to decide what to do for 7.1.</font><br /><font size="2">> If wedo nothing now, and then implement this feature in 7.2, we will</font><br /><font size="2">> have a backwards compatibilityproblem: EXECUTE 'SELECT INTO ...'</font><br /><font size="2">> will completely change in meaning.</font><br/><font size="2">></font><br /><font size="2">> I am inclined to keep our options open by forbiddingEXECUTE 'SELECT</font><br /><font size="2">> INTO ...' for now. That's more than a tad annoying, because thatleaves</font><br /><font size="2">> no useful way to do a dynamically-built SELECT, but if we don't forbid</font><br/><font size="2">> it I think we'll regret it later.</font><p><font size="2"> You can do somethinglike</font><p><font size="2"> FOR record_var IN EXECUTE <string-expr> LOOP</font><br /><font size="2"> ...</font><br /><font size="2"> END LOOP;</font><p><font size="2"> In this case, the <string-expr>executed over SPI_exec() must</font><br /><font size="2"> return tuples (0-n). Otherwise you'll geta runtime error.</font><p><font size="2"> Inside the loop you have access to the tuples via the record.</font><br /><fontsize="2"> Is that the dynamically-built SELECT capability you've been</font><br /><font size="2"> missing?</font><p><fontsize="2"> There's not that much need for mucking with temp tables in</font><br /><font size="2"> EXECUTE as all this discussion looks to me.</font><br /><p><font size="2">Jan</font><p><font size="2">--</font><p><fontsize="2">#======================================================================#</font><br /><fontsize="2"># It's easier to get forgiveness for being wrong than for being right. #</font><br /><font size="2"># Let'sbreak this rule - forgive me. #</font><br /><font size="2">#==================================================JanWieck@Yahoo.com #</font><br /><br /><p><font size="2">_________________________________________________________</font><br/><font size="2">Do You Yahoo!?</font><br /><fontsize="2">Get your free @yahoo.com address at <a href="http://mail.yahoo.com" target="_blank">http://mail.yahoo.com</a></font><code><fontsize="3"><br /><br /> **********************************************************************<br/> This email and any files transmitted with itare confidential and<br /> intended solely for the use of the individual or entity to whom they<br /> are addressed. Ifyou have received this email in error please notify<br /> Nick West - Global Infrastructure Manager.<br /><br /> This footnotealso confirms that this email message has been swept by<br /> MIMEsweeper for the presence of computer viruses.<br/><br /> www.mimesweeper.com<br /> **********************************************************************<br /></font></code>
Jan Wieck <janwieck@Yahoo.com> writes:
>> I am inclined to keep our options open by forbidding EXECUTE 'SELECT
>> INTO ...' for now. That's more than a tad annoying, because that leaves
>> no useful way to do a dynamically-built SELECT, but if we don't forbid
>> it I think we'll regret it later.
> You can do something like
> FOR record_var IN EXECUTE <string-expr> LOOP
> ...
> END LOOP;
Okay, that solves the concern I had about not being able to get the
result of an EXECUTEd select at all. I'll go ahead and forbid
EXECUTE 'SELECT INTO' for the time being, and we can talk about
improving plpgsql later.
regards, tom lane
Tom Lane writes: > I am inclined to keep our options open by forbidding EXECUTE 'SELECT > INTO ...' for now. That's more than a tad annoying, because that leaves > no useful way to do a dynamically-built SELECT, but if we don't forbid > it I think we'll regret it later. You can always use CREATE TABLE AS. -- Peter Eisentraut peter_e@gmx.net http://yi.org/peter-e/
Peter Eisentraut <peter_e@gmx.net> writes:
> Tom Lane writes:
>> I am inclined to keep our options open by forbidding EXECUTE 'SELECT
>> INTO ...' for now. That's more than a tad annoying, because that leaves
>> no useful way to do a dynamically-built SELECT, but if we don't forbid
>> it I think we'll regret it later.
> You can always use CREATE TABLE AS.
Does SPI_exec() support that? (Tries it ... seems to work ...)
Cool. OK, we have the bases covered then; there's no need to allow
SELECT INTO inside EXECUTE until we can make it work as expected.
regards, tom lane