PostgreSQL reports error 42P20 whenever a window function or window clause violates the SQL syntax rules that govern where window functions can appear or how their frames are defined. The parser and transformer reject invalid combinations such as window calls inside WHERE/JOIN filters, nested window functions, improper frame boundaries, or attempts to override a named window's partition/order definition.
The "42P20: windowing_error" SQLSTATE belongs to the syntax-error class 42. It is raised when the window-function parser or transformer detects a semantic violation in how a window function is used. Examples include placing a window function in disallowed contexts (JOIN conditions, WHERE, HAVING, LIMIT, policies, defaults, etc.), nesting window functions inside one another, defining a frame with a start that comes after its end, copying a named window that already has a frame, or giving a RANGE frame with offsetPRECEDING/FOLLOWING but supplying more than one ORDER BY column. PostgreSQL enforces these rules in parser modules such as parse_agg.c, parse_clause.c, and gram.y to remain compliant with SQL:2008 window-function semantics.
Read the error details shown by psql or your driver; PostgreSQL reports the clause name (SELECT, WHERE, JOIN, etc.) via ParseExprKindName and includes the position of the offending token. Use that location to isolate whether the window function lives inside a SELECT target list, WHERE filter, WINDOW clause, or JOIN condition.
Window functions are allowed only in SELECT target lists, ORDER BY, and the definition of a WINDOW clause itself. If you reference the result inside WHERE/HAVING/WHERE-like filters, push the window query into a subquery or CTE and apply the filtering to its output.
-- INVALID: window function inside WHERE
SELECT * FROM users WHERE ROW_NUMBER() OVER (ORDER BY created_at) = 1;
-- VALID: compute ranking first, then filter
WITH ranked AS (
SELECT *, ROW_NUMBER() OVER (ORDER BY created_at) AS rn
FROM users
)
SELECT * FROM ranked WHERE rn = 1;PostgreSQL rejects expressions where a window function argument contains another window function. Break the query into stages: compute the inner window function in a subquery or CTE, then reference that result when evaluating the outer aggregation.
-- INVALID
SELECT SUM(row_number() OVER (PARTITION BY dept ORDER BY salary)) OVER ()
FROM employees;
-- VALID
WITH ranks AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary) AS rn
FROM employees
)
SELECT SUM(rn) OVER () FROM ranks;Several parsing checks emit 42P20 for invalid frame clauses: frame start cannot be UNBOUNDED FOLLOWING, cannot end with CURRENT ROW when the start is already FOLLOWING, frame end cannot be UNBOUNDED PRECEDING, and GROUPS mode requires ORDER BY. RANGE frames with offset PRECEDING/FOLLOWING demand exactly one ORDER BY column with a valid ordering opfamily. Adjust the frame clause accordingly.
-- INVALID: frame starts after it ends and missing ORDER BY for GROUPS
SELECT avg(score) OVER (ORDER BY ts ROWS BETWEEN UNBOUNDED FOLLOWING AND CURRENT ROW) FROM events;
-- VALID
SELECT avg(score) OVER (ORDER BY ts ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) FROM events;
-- INVALID: GROUPS without ORDER BY
SELECT sum(score) OVER (GROUPS BETWEEN 1 PRECEDING AND CURRENT ROW) FROM events;
-- VALID
SELECT sum(score) OVER (ORDER BY ts GROUPS BETWEEN 1 PRECEDING AND CURRENT ROW) FROM events;When referencing a previously defined WINDOW clause, do not override its PARTITION BY or ORDER BY clauses and never wrap it in parentheses if the named window already has a frame. Either add your own window definition or name a different base window.
WINDOW base AS (PARTITION BY dept ORDER BY salary ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
SELECT *, SUM(score) OVER base FROM players; -- OK
SELECT *, SUM(score) OVER (base ORDER BY salary) FROM players; -- INVALID: cannot override ORDER BY
SELECT *, SUM(score) OVER (base) FROM players; -- INVALID if base already has a frameAfter each refactor, rerun the query to ensure the parser accepts the window definition before adding more clauses. PostgreSQL returns distinct hints for each violation, so fixing one error often reveals the next restriction quickly.
Windowing_error is a parser-time guard that keeps PostgreSQL compliant with SQL:2008 window semantics. The error surfaces from modules such as parse_agg.c (which checks ParseExprKind, forbidding window functions in JOIN/WHERE/HAVING/policy/default/index expressions), parse_clause.c (which handles named WINDOW clauses and frame validation), and gram.y (which enforces legal frame boundaries). At the executor level, execExpr.c also raises 42P20 when it encounters nested window functions while initializing expressions. When migrating from databases that allow looser window behavior, these restrictions are often the first place where PostgreSQL stops queries before execution.
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