Sooner or later every web developer comes across the situation when the site “has gone down” and must be “got back to life”. The reasons for that may be diverse, but in this particular article we’ll elaborate on one certain cure that's capable of increasing Drupal productivity, i.e. INDICES.
What are indices?
Indices are applied for quick search of lines that contain one indicated column value. Without an index function the whole table is being read through from the top data record until the relevant lines will be found. The bigger the table, the bigger the expenses. In the case of the table containing an index for the required column, MySQL can identify the search position in the middle of data file without leafing through the whole bulk of data. In the case of a table consisting of 1000 lines, the search will be at least 100 times faster as compared to successive examination of all notes. But if an access to all 1000 lines is needed, the successive reading appeares to be faster since the disc search operation is not required (source).