[Solved] Performance of join vs pre-select on MsSQL

EverSQL Database Performance Knowledge Base

Performance of join vs pre-select on MsSQL

Database type:

I can do the same query in two ways as following, will #1 be more efficient as we don't have join?


select table1.* from table1 
inner join table2 on table1.key = table2.key 
where table2.id = 1


select * from table1 
where key = (select key from table2 where id=1)

How to optimize this SQL query?

The following recommendations will help you in your 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 table1_idx_key ON table1 (key);
CREATE INDEX table2_idx_id ON table2 (id);
The optimized query:
            ON table1.key = table2.key 
        table2.id = 1

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* original question posted on StackOverflow here.