title: Will a newly added index to a table with a large number of rows have an immediate effect on queries?
body: <p>I recently added some non unique indexes to two tables. Theses tables have number of rows in the millions, and 100mbs in size (including the size of indexes). After adding the new INDEX type index (none unique) one of the index had a cardinality comparable to that of the primary key, the other column was a rather low cardinality. Correct me if I'm wrong but my understanding is that the higher the cardinality of an index the more useful it will be in speeding where clause queries. So since one had a high cardinality I though this should have a noticeable effect on queries. </p>
  
  <p>Anyway after adding the indexes, I was not sure if I saw a difference or not in the speed of the where queries using those columns. Then I somewhat recall that in the past when I added indexes it took some time before it effected queries (like a day or two). Maybe it was just my imagination. Will the newly added indexes have immediate effect in this case? In any case?</p>
  
  <p>Thank you for your time.</p>