As has been mentioned there is no fixed answer to this. It all depends on the particular context. But just for the sake of an answer. Take this simple query:

 SELECT first_name FROM people WHERE last_name = 'Smith';


To process this query without an index, every column, last_name must be checked for every row in the table (full table scan). 

With an index, you could just follow a B-tree data structure until 'Smith' was found.

With a non index the worst case looks linear (n), whereas with a B-tree it would be log n, hence computationally less expensive.

Performance of SQL query with condition vs. without where clause
The pirate song “Fifteen Men on a Dead Man’s Chest” from Treasure Island contains the expression yo-ho-ho.


  Fifteen men on the dead man’s chest—
  Yo-ho-ho, and a bottle of rum!
  Drink and the devil had done for the rest—
  Yo-ho-ho, and a bottle of rum!


Does this signify laughter, a piratical variation of ho-ho-ho? It doesn’t seem like an amusing little ditty but then pirates probably would have a dark sense of humour. Or is it simply a more piratey song filler than na-na-na?
