Database New Added 9 September 2026

Redis: WRONGTYPE Operation against a key holding the wrong kind of value

The key exists but holds a different type from the one the command expects, for instance GET against a hash. In practice it means two parts of the system disagree about the shape of a key, usually because a prefix is missing and two features have collided on the same name.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# What is actually stored there?
redis-cli type session:42
redis-cli object encoding session:42

# Use the command that matches the type
redis-cli hgetall session:42     # not GET

# Namespace keys so two features cannot collide
SET app:v2:session:42 ...

# Find the writer that disagrees
redis-cli --scan --pattern 'session:*' | head
redis-cli monitor | grep session:42     # noisy, use for a few seconds

# A migration that changed a key's type needs the old keys removed

How to diagnose Database errors

Database errors group into connection exhaustion, lock contention and deadlocks, constraint violations, and resource limits. Connection errors under load are the most misdiagnosed: "too many connections" is almost never fixed by raising max_connections, because each connection costs memory. It is fixed by putting a pooler in front and finding the code path that leaks connections.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Look at what the server currently sees: SELECT * FROM pg_stat_activity in PostgreSQL, SHOW FULL PROCESSLIST in MySQL. Sessions sitting in idle in transaction are the usual culprit behind connection exhaustion.
For deadlocks, read the deadlock report the database logs. It names both transactions and the exact lock order. The fix is nearly always to make all code paths acquire locks in the same order.
Check whether the error is from the database or from a pooler. PgBouncer, ProxySQL and RDS Proxy return their own errors that look database-native but need pooler-side fixes.
Run EXPLAIN (ANALYZE, BUFFERS) on slow queries. Lock-wait timeouts are often just very slow queries holding locks longer than they should.
Verify you are connected to the writer, not a read replica. READONLY and "cannot execute in a read-only transaction" errors mean traffic is reaching the wrong endpoint.

Tools worth reaching for

pg_stat_activity
SHOW ENGINE INNODB STATUS
EXPLAIN ANALYZE
slow query log
pgbadger

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

redis.io

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.