Database New Added 8 September 2026

PostgreSQL: cannot execute INSERT in a read-only transaction

The connection landed on a standby, or on a primary that has been demoted or put into read only mode. After a failover an application pooling connections keeps writing to the old endpoint until something forces it to reconnect, which is why writes fail while reads look healthy.

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

-- Which server is this, and is it in recovery?
SELECT pg_is_in_recovery(), inet_server_addr(), current_setting('default_transaction_read_only');

# Route writes at the primary explicitly
postgresql://host1:5432,host2:5432/app?target_session_attrs=read-write

-- Someone set it deliberately
ALTER DATABASE app SET default_transaction_read_only = off;

# After a failover, recycle the pool so it stops using stale sockets
# SQLAlchemy: pool_pre_ping=True, pool_recycle=300

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.

postgresql.org

Related Database errors

See all 41 Database errors →

Browse other categories

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.