All Oracle errors
ORA-01555transaction

Snapshot too old

ORA-01555: snapshot too old: rollback segment number not large enough

Why it happens

READ-consistent read-timeout: another session overwrote rollback data that a long-running or fetch-across-commit query still needs.

How to fix it

Avoid committing inside loops over fetched rows, size UNDO_RETENTION/undo tablespace higher, batch smaller, and do not use DML over the same tables the query reads.

A note on ORA-01555

This entry is part of a hand-curated set of ORA-, FRM-, REP- and TNS- codes that actually turn up in production Oracle estates — SQL*Plus, APEX, 19c/21c, ODP.NET apps, Oracle Forms 6i/12c and Reports included. The message text above is the canonical wording; the cause and fix are written in plain language so a developer or DBA can act without digging through pages of Oracle reference docs first.

Codes outside this list of 44 are far less common. If you hit one, the official Oracle error reference and your instance's alert or listener log remain the authoritative source.

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