Question

This is more of a correctness question. Say I have a table with a primary key column in my database. In my DAO code I have a function called insertRow(string key) that will return true if the key doesn’t exist in the table and insert a new row with the key. Otherwise, if a row already exists with that key it returns false. Is it better/worse to have insertRow first check for the existence of the key or just go ahead and do the insert and catch the duplicate key error? Or is saving on a single select statement too trivial an optimization to even bother worrying about?

So in sudo code:

boolean insertRow(String key){
    //potentially a select + insert
    if(select count(*) from mytable where key = "somekey" == 0){
       insert into mytable values("somekey")
       return true;
    }
    return false;
}

or

  boolean insertRow(String key){
    try{
       //always just 1 insert
       insert into mytable values("somekey")
       return true;
    } catch (DuplicateKeyException ex){}
    return false;
  }

Answer 1

Try the insert, then catch the error.

Otherwise, you could still have a concurrency issue between two active SPIDs (lets say two web users on the system at the same time), in which case, you’d have to catch the error anyway:

User1: Check for key "newkey"? Not in database.
User2: Check for key "newkey"? Not in database.
User1: Insert key "newkey". Success.
User2: Insert key "newkey". Duplicate Key Error.

You can mitigate this by using explicit transactions or setting the transaction-isolation level, but its just easier to use the second technique, unless you are sure only one application thread is running against the database at all times.