PREPARE placeholder type coercion fails when plain queries work fine

[cetra3]

Describe the bug

Discovered this one while playing around with prepared statements. The type coercion of placeholders is different between a PREPARE statement and the normal query.

I.e,

The standard select query:

SELECT $1 = 'x', $1 = s FROM t

And

The prepared statement:

PREPARE p AS SELECT $1 = 'x', $1 = s FROM t

Both diverge in their placeholder resolution, and can fail in strange ways

To Reproduce

This only appears when the types are potentially different. I.e, I saw it when the type on the table was Utf8View but literal strings are Utf8.

I did manage to replicate it using the datafusion cli.

I.e, first create a table definition (ensuring map_string_types_to_utf8view is on)

CREATE TABLE t(s VARCHAR) AS VALUES ('a');

Do the raw query (works fine):

SELECT $1 = 'x', $1 = s FROM t;

Prepare this query:

PREPARE p AS SELECT $1 = 'x', $1 = s FROM t;

This will error with:

Error during planning: Expected parameter of type Utf8, got Utf8View: Conflicting types for id $1

Expected behavior

I'd expect this to work fine

Additional context

No response

[RatulDawar]

take

[RatulDawar]

@alamb would it make sense to use comparison_coercion in get_parameter_fields() when merging placeholder types, instead of strict equality? That would unify cases like Utf8 + Utf8View → Utf8View, matching what normal queries already do at comparison sites, while still erroring on truly incompatible types.