Definition

Write skew is an anomaly where two concurrent transactions each read overlapping data and then write non-overlapping fields, resulting in an overall invalid state despite no single column being overwritten.

Core concept

Occurs under snapshot isolation. Example: two doctors scheduling themselves off on same day, checking each other’s availability and both writing an appointment because they didn’t see the other’s write at start.

Use cases

Snapshot isolation can allow write skew. Serializable isolation prevents it.

Example

Table of on-call doctors with boolean fields. Both T1 and T2 see the other is on-call, each marks themselves off. Both commit, now no doctor is on-call (invalid).

References

Known concept in transaction anomalies.