ORA-01555 Snapshot Too Old (Eski Sürüm Üzerine Yazılma Hatası)

The infamous Oracle error raised when a long-running query requires a historical block version whose undo record has been overwritten due to expired undo retention.

Internal Architecture Breakdown

Oracle guarantees read consistency anchored to the query's start SCN. If concurrent transactions modify data and overwrite inactive undo blocks before a long-running batch job finishes reading them, the engine aborts with ORA-01555.

Remediation requires sizing the UNDO tablespace adequately, extending UNDO_RETENTION, enabling RETENTION GUARANTEE, or tuning slow full-table scans.

Production Inspection Queries

Oracle

SELECT maxquerylen, ssolderrcnt, undoblks FROM v$undostat WHERE ssolderrcnt > 0;
SHOW PARAMETER undo_retention;

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