title: I have million rows in one table. I am tolde to use union instead on in clause for performance reason. is it true?
body: <p>We have million rows in one table.</p>
  
  <p>our select:</p>
  
  <pre><code>select * from tableA where column1 in ("a", "b", "c", "d") and where column2 = "abc";
  </code></pre>
  
  <p>and we have unique index on column1 and column2 combined.</p>
  
  <p>I was told to swtich to :</p>
  
  <pre><code>select * from tableA where column1 = "a" and column2 = "abc"
  union
  select * from tableA where column1 = "b" and column2 = "abc"
  union
  select * from tableA where column1 = "c" and column2 = "abc"
  union
  select * from tableA where column1 = "d" and column2 = "abc";
  </code></pre>
  
  <p>We could have from 1 to 100 different values in IN clause. So is it better to run one statement with IN clause or run 100 statement and perform union.</p>