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#1PG Bug reporting formPG Bug reporting form
Oct 16, 22:51
#2Venu K↳Venu Kto PG Bug reporting form (#1)
Oct 16, 23:06
#3Venu K↳Venu Kto Venu K (#2)
1 day ago

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

Started by PG Bug reporting form8 days ago3 messagesbugs
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You can run a PostgreSQL built from this patch straight from Docker, with no checkout and no build:

docker run --rm -p 5432:5432 ghcr.io/hackorum-dev/postgres-patch:t254247
psql -h localhost -U postgres

Built from patchset v2 (message #2), October 24, 2026 at 06:16 AM.

Every patchset is also pushed to a branch of our PostgreSQL fork, so you can check out the same tree CI built. Without a PostgreSQL checkout:

git clone --branch t254247_2 https://github.com/hackorum-dev/postgres.git

In a checkout you already have, add the fork once:

git remote add hackorum https://github.com/hackorum-dev/postgres.git

then, for this patchset and every later one:

git fetch hackorum t254247_2 && git checkout t254247_2

Patchset v2 (message #2) is on t254247_2

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#1PG Bug reporting form
PG Bug reporting form
noreply@postgresql.org

The following bug has been logged on the website:

Bug reference: 19768
Logged by: Venu Kummari
Email address: venu2805k@gmail.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.

#2Venu K
Venu K
venu2805k@gmail.com
In reply to: PG Bug reporting form (#1)
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@postgresql.org> wrote:

Show quoted text

The following bug has been logged on the website:

Bug reference: 19768
Logged by: Venu Kummari
Email address: venu2805k@gmail.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.

Attachments:

t254247_2
v1-0001-Assign-collations-to-JSON_TABLE-s-context-item-an.patchapplication/octet-stream; name=v1-0001-Assign-collations-to-JSON_TABLE-s-context-item-an.patchDownload+40-1
#3Venu K
Venu K
venu2805k@gmail.com
In reply to: Venu K (#2)
Re: BUG #19768: JSON_TABLE: row/text comparisons in context item or PASSING give wrong results or spurious collation

Hi,

Gentle ping on this. The bug still reproduces on current master
(126976a), and the patch upthread still applies cleanly there. The
one-line fix in parse_jsontable.c also applies to REL_17_STABLE and
REL_18_STABLE; the regression test hunks need minor adjustment on
those branches.

CC'ing Amit as the committer of JSON_TABLE.

Thanks,
Venu

Show quoted text

On Sat, Oct 17, 2026 at 4:36 AM Venu K <venu2805k@gmail.com> wrote:

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@postgresql.org> wrote:

The following bug has been logged on the website:

Bug reference: 19768
Logged by: Venu Kummari
Email address: venu2805k@gmail.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.

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