[Solved] Optimising a SELECT query that runs slow on Oracle which runs quickly on SQL Server

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. 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.
  2. Replace In Subquery With Correlated Exists (modified query below): In many cases, an EXISTS subquery with a correlated condition will perform better than a non correlated IN subquery.
Optimal indexes for this query:
CREATE INDEX tasks_idx_orderid_engineer1 ON tasks (orderID,engineer1);
The optimized query:
SELECT
        tasks.orderID 
    FROM
        tasks 
    WHERE
        NOT EXISTS (
            SELECT
                DISTINCT 1 
            FROM
                tasks AS tasks1 
            WHERE
                (
                    tasks1.engineer1 IS NOT NULL 
                    AND tasks1.engineer2 IS NOT NULL
                ) 
                AND (
                    tasks.orderID = tasks1.orderID
                )
        )

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* original question posted on StackOverflow here.