Incident brief
Collation mismatch (explicit collations)
A comparison cannot combine two different explicit COLLATE clauses; applying 'C' to one side and 'POSIX' to the other raises this collation-mismatch error.
In 10 seconds
- What
- Collation mismatch (explicit collations)
- What triggers it
- Compare two expressions where each side carries a different explicit COLLATE clause.
- The fix
- Use a single explicit collation for the comparison, or none and let the column defaults apply.
- Proof
- Reproduced on PostgreSQL 18.4 → A single query reproduces SQLSTATE 42P21; the two conflicting explicit collations cannot be combined.
The fix
What to do right now
The immediate, application-level response to this error.
- Use a single explicit collation for the comparison, or none and let the column defaults apply.
- Apply one COLLATE to the whole comparison rather than a different one to each side.
-- Use one collation (or the column defaults) for the comparison.
SELECT label = note FROM coll_demo;Diagnose
See it live on the server
Run these against the affected instance to confirm the diagnosis before you act.
Standard triage — not specific to this error
These are canonical PostgreSQL system-catalog queries, shown as SQL to run. No sample output is attached because this is general triage, not a captured lab transcript.This SQLSTATE does not have an error-specific live snapshot yet. These are the canonical system-catalog queries you run against the affected server to see the problem in real time — standard triage, not a reproduced transcript.
What is running right now
Active backends, how long each has been running, and what it is waiting on.
SELECT pid,
state,
wait_event_type,
wait_event,
now() - query_start AS running_for,
left(query, 80) AS query
FROM pg_stat_activity
WHERE state <> 'idle'
AND pid <> pg_backend_pid()
ORDER BY running_for DESC NULLS LAST;Who is blocking whom
Turn raw blocking PIDs into the actual queries on both sides of the wait.
SELECT blocked.pid AS blocked_pid,
blocked.query AS blocked_query,
blocking.pid AS blocking_pid,
blocking.query AS blocking_query
FROM pg_stat_activity AS blocked
JOIN LATERAL unnest(pg_blocking_pids(blocked.pid)) AS b(pid) ON true
JOIN pg_stat_activity AS blocking ON blocking.pid = b.pid
WHERE cardinality(pg_blocking_pids(blocked.pid)) > 0;Locks that are still waiting
Every lock a backend has requested but not yet been granted.
SELECT l.pid,
l.locktype,
l.mode,
l.granted,
COALESCE(c.relname, l.transactionid::text) AS object
FROM pg_locks l
LEFT JOIN pg_class c ON c.oid = l.relation
WHERE NOT l.granted
ORDER BY l.pid;Why it happens
What PostgreSQL is telling you
The mechanism behind the error, grounded in the official manual — not paraphrased.
PostgreSQL 18 Documentation — Appendix A. PostgreSQL Error Codes (Table A.1, Class 42 — Syntax Error or Access Rule Violation)
42P21 → collation_mismatchRead the full section on postgresql.org →
Combining two explicit collations
One side is COLLATE 'C' and the other COLLATE 'POSIX', so PostgreSQL raises 'collation mismatch between explicit collations C and POSIX'.The session continues normally
The failing statement ran outside a transaction block, so nothing was left in a bad state — the next statement in the same connection runs normally.Reproduce & verify
A real, single-session PostgreSQL reproduction
A literal transcript of SQL run against a live PostgreSQL instance in an isolated lab — the commands below are exactly what was executed.
- 1Compare two expressions where each side carries a different explicit COLLATE clause.
- 2PostgreSQL tries to reconcile the two explicit collations for the comparison.
- 3Because they conflict, the statement aborts with SQLSTATE 42P21.
A comparison hand-applied different COLLATE clauses to each side and PostgreSQL refused to guess which one wins.
CREATE TABLE coll_demo(label text, note text);
INSERT INTO coll_demo VALUES ('a','b');SELECT (label COLLATE "C") = (note COLLATE "POSIX") FROM coll_demo;SELECT 'ok' AS session_after_error;What PostgreSQL actually returned
CREATE TABLE
INSERT 0 1ERROR: collation mismatch between explicit collations "C" and "POSIX"
LINE 1: SELECT (label COLLATE "C") = (note COLLATE "POSIX") FROM col...
^ session_after_error
---------------------
ok
(1 row)The comparison runs once it uses a single, consistent collation.
We compare the columns without conflicting explicit collations to show a clean result.
Without this
Before: two explicit collations conflict
With this, tested
After: a single collation compares cleanly
What Pro unlocks here
- The exact prevention SQL — copy-paste ready
- Raw psql output captured from the Docker lab
- A senior-DBA action list to take it further
- Live monitoring queries to catch it in production
- The deeper audit: fix-that-fails counterexample, GUC before/after, server-log evidence
Related & next steps
Follow the thread
Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.
Verification
- Last verified
- 2026-07-24 (isolated lab, PostgreSQL 18.4)
- Reviewed by
- Verified against PostgreSQL 18.4 in an isolated lab environment
- Audit status
- reviewed