Why Does My Oracle UPDATE Query with a JOIN Hang, and How Can I Fix It?

Troubleshooting an Indefinite Update Query with Join in Oracle

When executing the following UPDATE query:

UPDATE table1 t1 SET (t1.col,t1.Output) = (
  SELECT t2.col, t3.Output + t2.col
  FROM tabl2 t3 
  LEFT JOIN table1 t2 ON t3.Join_Key = t2.Join_Key
  WHERE t2.col is not NULL);

users may encounter an indefinite execution error. This article delves into the issue and provides a solution.

The Problem: Non-Unique Subquery Results

The root cause of the error lies in the subquery within the UPDATE statement. Unless the subquery returns a single row for each row in table1, the update operation will fail. The LEFT JOIN in the subquery allows for multiple matching rows, potentially leading to a non-deterministic update.

Solution: Ensuring Single-Row Subquery Returns

To resolve the issue, a condition is needed to relate rows in table1 to rows in the subquery, ensuring that the subquery returns a single row. This can be achieved by adding a join condition that matches rows by a unique key:

UPDATE table1 t1 SET (t1.col,t1.Output) = (
  SELECT t2.col, t3.Output + t2.col
  FROM tabl2 t3 
  LEFT JOIN table1 t2 ON t3.Join_Key = t2.Join_Key
  WHERE t2.col is not NULL
    AND t1.some_key = t2.some_key);

Addressing Scope of the Update

Another consideration is the fact that the UPDATE statement may affect all rows in table1. It is important to evaluate whether this is the intended behavior or if a more selective update is necessary. By including additional WHERE conditions in the UPDATE statement, users can limit the scope of the update to specific rows, as needed.

The above is the detailed content of Why Does My Oracle UPDATE Query with a JOIN Hang, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!