Re: [PL/PgSQL] EXECUTE...USING enhancement proposal

2010/1/14 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
> Hello
>
> I thing, so this is bad idea.
>
> a) this behave depends on DDL implementation, not plpgsql implementation
>
> b) proposed implementation needs some escape magic. This was first
> implementation of USING clause and it was rejected. Some composite and
> nested values are significant break.
>
> see in archive http://archives.postgresql.org/pgsql-patches/2006-08/msg00267.php
>
> Regards
> Pavel Stehule
>
>
> 2010/1/14 Vincenzo Romano <vincenzo(dot)romano(at)notorand(dot)it>:
>> Hi all.
>> There's currently a limitation in the v8.4.2 implementation of the
>> EXECUTE...USING predicate in PL/PgSQL which prevents you from
>> exploiting the USING-supplied value list with DDL commands.
>> For example:
>>
>> CREATE TABLE test ( i int );
>> ...
>> EXECUTE 'ALTER TABLE test ALTER COLUMN i SET DEFAULT $1' USING 42;
>>
>> complains with:
>>
>> ERROR:  there is no parameter $1
>> CONTEXT:  SQL statement "ALTER TABLE test ALTER COLUMN i SET DEFAULT $1"
>>
>> while:
>>
>> EXECUTE 'SELECT $1' USING 42;
>>
>> works.
>> In both cases the $1 variable/placeholder refers to a constant value.
>> And actually, even if the "thing" defined after the USING lexeme was a
>> variable, that should be evaluated and substituted *before* executing
>> the command.
>>
>> The current documentation
>> (http://www.postgresql.org/docs/8.4/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN)
>> doesn't say so and clearly describes how this feature is meant to
>> work.
>> Quoting:
>> ----
>> The command string can use parameter values, which are referenced in
>> the command as $1, $2,
>> etc. These symbols refer to values supplied in the USING clause. This
>> method is often preferable to
>> inserting data values into the command string as text: it avoids
>> run-time overhead of converting the
>> values to text and back, and it is much less prone to SQL-injection
>> attacks since there is no need for
>> quoting or escaping. An example is:
>> ----
>> (38.5.4. Executing Dynamic Commands)
>>
>> It talks about "values", that is typed constants.
>> Please, refer also to the following discussion on pgsql-general mailing list:
>> http://archives.postgresql.org/pgsql-general/2010-01/msg00522.php
>>
>> My proposal is to relax that (clearly running but undocumented)
>> constraint and allow any SQL command in the EXECUTE...USING predicate.
>> I would leave the responsibility to the programmer to ensure whether
>> the dynamic command makes any syntactic and semantic sense.
>>
>> --
>> Vincenzo Romano
>> NotOrAnd Information Technologies
>> NON QVIETIS MARIBVS NAVTA PERITVS

Well, the very basic proposal could then be changed into:

Allow some kind of syntactic replacement of the placeholders found
into the command string with values taken from the USING clause
evaluated straight into the PLPGSQL function body environment. The
model could be the C language sprintf().

Maybe you can think about using different placeholders for "static"
(or "local") evaluation, like #1, #2 ... #n.

For example, you could do something like this:

EXECUTE 'ALTER TABLE '||tablename||' ADD CHECK col1=#1 and col2>=#2
and col2<#3" USING currval1, currval2, currval3.

The execution, within the PLPGSQL interpreter, would proceed like this:

0. Concatenate the sub-strings to just 1.
1. Evaluate the variable list after the USING clause (currval1,
currval2, currval3) to their current values.
2. Replace the placeholders with the natural ordered references within
the command string
3. Send the final string to the execution.

This makes a lot of sense (in my opinion) for higher level functions
(functions which create functions which execute dynamic commands).
It's more like a string substitution but with knowledge of the syntac
of the expressions following the USING clause.

--
Vincenzo Romano
NotOrAnd Information Technologies
NON QVIETIS MARIBVS NAVTA PERITVS