What is mutating table error?

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Mutating table error is occurred when a trigger tries to update a row that it is currently using. It is fixed by using views or temporary tables.

In PL/SQL, a mutating table error occurs when a trigger or stored procedure references a table that is currently being modified by the statement that invoked the trigger or procedure. The error is raised to prevent actions that could result in inconsistent or undefined data.

Here’s a more detailed explanation:

  1. Trigger Context: When a row-level trigger (BEFORE INSERT, AFTER UPDATE, etc.) is fired, the trigger cannot query or modify the table that the trigger is associated with. This is because the trigger is part of the same transaction that is currently modifying the table, and any changes made in the trigger could lead to inconsistencies.
  2. Stored Procedure Context: Similarly, if a stored procedure is modifying a table and that stored procedure contains a query on the same table it is modifying, a mutating table error can occur.

To resolve this issue, developers typically use a combination of triggers and variables to store values needed for subsequent logic, or they use a compound trigger (available in Oracle Database 11g and later) to handle multiple trigger events in a single execution context.

In summary, the mutating table error is a mechanism in PL/SQL to prevent conflicts that could arise when a trigger or procedure attempts to query or modify a table that is currently being modified by the statement that invoked the trigger or procedure.