But may not see the "current" version of the database.

E.g. T₁ does select, then concurrent T₂ deletes some of T₁'s selected tuples

This is OK unless tx's communicate outside the database system.

E.g. T₁ counts tuples while T₂ deletes then counts; then counts are compared

Use locks if application needs every tx to see same current version

• LOCK TABLE locks an entire table
• SELECT FOR UPDATE locks only the selected rows