[Solved] Performance of two left joins versus union

How to optimize this SQL query?

In case you have your own slow SQL query, you can optimize it automatically here.

For the query above, the following recommendations will be helpful as part of the SQL tuning process.
You'll find 3 sections below:

  1. Description of the steps you can take to speed up the query.
  2. The optimal indexes for this query, which you can copy and create in your database.
  3. An automatically re-written query you can copy and execute in your database.
The optimization process and recommendations:
  1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
  2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
Optimal indexes for this query:
CREATE INDEX table2_idx_table1id ON Table2 (Table1ID);
CREATE INDEX table3_idx_table1id ON Table3 (Table1ID);
The optimized query:
        dbo.Table1 T 
        Table2 T2 
            ON T.ID = T2.Table1ID 
        Table3 T3 
            ON T.ID = T3.Table1ID 
        T2.Table1ID IS NOT NULL 
        OR T3.Table1ID IS NOT NULL

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* original question posted on StackOverflow here.