Thread: BUG #17051: Incorrect params inferred on PREPARE

The following bug has been logged on the website:

Bug reference:      17051
Logged by:          Arthur McGibbon
Email address:      arthur.mcgibbon@gmail.com
PostgreSQL version: 13.3
Operating system:   Windows 10 + WSL2 + Docker
Description:

Using the table...

  CREATE TABLE testSchema.testTable (timestampCol timestamp);

...and preparing the query...

  PREPARE testQuery (unknown) AS
  UPDATE testSchema.testTable
  SET timestampCol = CASE WHEN timestampCol IS NULL THEN $1 ELSE NULL END;

...results in an error...

  ERROR:  column "timestampcol" is of type timestamp without time zone but
expression is of type text
  LINE 3:   set timestampCol = case when timestampCol is null then $1 ...
                             ^
  HINT:  You will need to rewrite or cast the expression.
  SQL state: 42804
  Character: 80

Specifying the parameter as timestamp works without error...
  PREPARE testQuery (timestamp) AS
  UPDATE testSchema.testTable
  SET timestampCol = CASE WHEN timestampCol IS NULL THEN $1 ELSE NULL END;

I'd hope that PostgreSQL would infer a "timestamp" type here or reply with
an error that it couldn't infer the parameter type.
Why does it default to type "text" and then effectively say the query is
invalid?

I can cast my parameter in the query to get around this but am submitting
the bug because inferring "text" seems wrong.

I'd hope that PostgreSQL would infer a "timestamp" type here or reply with
an error that it couldn't infer the parameter type.
Why does it default to type "text" and then effectively say the query is
invalid?

I can cast my parameter in the query to get around this but am submitting
the bug because inferring "text" seems wrong.

Could you point me to the documentation - I could only find https://www.postgresql.org/docs/current/sql-prepare.html and it doesn't mention how unknown params are handled.

"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Tuesday, June 8, 2021, Arthur McGibbon <arthur.mcgibbon@gmail.com> wrote:
>> Could you point me to the documentation - I could only find
>> https://www.postgresql.org/docs/current/sql-prepare.html and it doesn't
>> mention how unknown params are handled.

> https://www.postgresql.org/docs/current/typeconv-union-case.html

Yeah, the point here is that the type of the parameter symbol is
guessed in the context of resolving the CASE construct.  There's
no mechanism for applying external knowledge about what that CASE
ought to yield.  While maybe we could do something in this
specific context, examples that are only slightly more complex
would really be quite impossible.  For example,

... SET timestampCol = foo(CASE WHEN timestampCol IS NULL THEN $1 ELSE NULL END);

There's no way to tell which foo() function is meant until we've
identified a result type for the CASE, so the function context is
pretty much a blocker.

Roughly speaking, we only get to use one syntactic level of context
to guess the type of a parameter symbol.  Doing better would really
be a research project.

            regards, tom lane