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Subject: BUG #19731: `first_value` returns an excluded peer for a nonempty
 `EXCLUDE TIES` frame
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: theshallow27@gmail.com
Reply-To: theshallow27@gmail.com, pgsql-bugs@lists.postgresql.org
Date: Tue, 29 Sep 2026 19:16:36 +0000
Message-ID: <19731-bf41225d46718e0d@postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19731
Logged by:          Shallow
Email address:      theshallow27@gmail.com
PostgreSQL version: 18.6
Operating system:   Linux
Description:       =20

Summary

When a `ROWS` frame does not include the current row, PostgreSQL can make
`first_value` return a peer excluded from the frame. In this reproducer, the
first row's frame contains only `1` after `EXCLUDE TIES`, and `array_agg`
confirms that frame, but `first_value` returns `0`.

## Reproducer

```sql
WITH t(x) AS (
  VALUES (0), (0), (1)
)
SELECT x,
       first_value(x) OVER w AS first_in_frame,
       array_agg(x) OVER w AS frame_values
FROM t
WINDOW w AS (
  ORDER BY x
  ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING
  EXCLUDE TIES
);
```

The SQL window frame is nonempty, so `first_value` should return its first
value. `EXCLUDE TIES` removes peers of the current row but retains non-peer
rows; it must not cause an excluded peer to be returned as though it were
still in the frame.
The related `last_value` case also fails for a frame ending before the
current row. The local triage identifies `WinGetFuncInFrame` in
`src/backend/executor/nodeWindowAgg.c` as the common path: its
current-position fallback appears to assume the current row is in the frame.