During concurrent transactions that involve select queries and update commands, pseudo column-level locking can allow some queries to return values from locked rows, and can allow other queries to avoid blocking on locked rows that do not qualify. Pseudo column-level locking can reduce blocking:
When the select query does not reference columns on which there is an uncommitted update.
When the where clause of a select query references one or more columns affected by an uncommitted update, but the row does not qualify due to conditions in other clauses.
When neither the old nor new value of the updated column qualifies, and an index containing the updated column is being used.