Re: BUG #19768: JSON_TABLE: row/text comparisons in context item or PASSING give wrong results or spurious collation

Hello,

Please find attached a proposed patch for BUG #19768.

The patch adds the missing collation assignment in JSON_TABLE expression
transformation and includes regression tests for the reported incorrect
row-comparison result and collation-resolution errors.

The patch was tested on master, and the core regression suite passed
(239/239 tests).

Regards,
Venu Kummari

On Sat, Oct 17, 2026 at 4:21 AM PG Bug reporting form <
noreply(at)postgresql(dot)org> wrote:

> The following bug has been logged on the website:
>
> Bug reference: 19768
> Logged by: Venu Kummari
> Email address: venu2805k(at)gmail(dot)com
> PostgreSQL version: 18.6
> Operating system: Linux x86_64
> Description:
>
> PostgreSQL versions: 17.11, 18.6, master (26bd08f). Tested from source on
> Linux x86_64.
>
> The JSON_TABLE context-item expression fails to assign collations
> correctly,
> causing incorrect results for row comparisons and collation errors for
> valid
> text comparisons.
>
> 1. Incorrect result / assertion failure:
>
> SELECT * FROM JSON_TABLE(
> jsonb_build_object('k', ROW(1, 2) < ROW(3, 4)),
> '$'
> COLUMNS (a bool PATH '$.k')
> );
>
> Expected: t
> Actual: f on PostgreSQL 17.11 and 18.6. On an assertion-enabled master
> build, the backend fails with:
>
> TRAP: failed Assert("list_length(rcexpr->inputcollids) == nopers"), File:
> "execExpr.c", Line: 2078
>
> 2. Collation error in the context item:
>
> SELECT * FROM JSON_TABLE(
> jsonb_build_object('k', 'a' < 'b' COLLATE "C"),
> '$'
> COLUMNS (a bool PATH '$.k')
> );
>
> Expected: t
> Actual: ERROR: could not determine which collation to use for string
> comparison
>
> 3. Collation error in a PASSING argument:
>
> SELECT * FROM JSON_TABLE(
> jsonb '{}',
> '$' PASSING 'a' < 'b' COLLATE "C" AS x
> COLUMNS (a bool PATH '$x')
> );
>
> This also fails with "could not determine which collation to use for string
> comparison", despite the explicit COLLATE clause.
>
> The same expressions work outside JSON_TABLE.
>
> Suspected cause: transformJsonTable() in
> src/backend/parser/parse_jsontable.c does not call assign_expr_collations()
> for the context-item expression and PASSING arguments. The corresponding
> XMLTABLE transformation performs collation assignment, as do JSON_TABLE
> column expressions.
>
> Without the required collation information, a RowCompareExpr can have an
> empty inputcollids list, causing the executor to compare zero columns and
> return an incorrect result.
>
> A proposed patch adds the missing collation assignment and regression
> tests.
> The patch was tested on master, and the core regression suite passed
> (239/239 tests). The source change applies to REL_17_STABLE and
> REL_18_STABLE.
>
> The bug was discovered through grammar-based fuzzing.
>
>
>