Indexed Lookups from TIMESTAMP Columns (’2018-10-28 00:30:00’), (’2018-10-28 01:30:00’); mysql>  SELECT ts FROM tstable; +---------------------+ | ts | +---------------------+ | 2018-10-28 00:30:00 | | 2018-10-28 01:30:00 | +---------------------+ mysql>  SET time_zone = ’MET’; -- retrieve non-UTC values mysql>  SELECT ts FROM tstable; +---------------------+ | ts | +---------------------+ | 2018-10-28 02:30:00 | | 2018-10-28 02:30:00 | +---------------------+ To use named time zones such as  ’MET’  or  ’Europe/Amsterdam’ , the time zone tables must be properly set up. For instructions, see Section 5.1.15, “MySQL Server Time Zone Support” . You can see that the two distinct UTC values are the same when converted to the  ’MET’  time zone. This phenomenon can lead to different results for a given  TIMESTAMP  column query, depending on whether the optimizer uses an index to execute the query. Suppose that a query selects values from the table shown earlier using a  WHERE  clause to search the  ts column for a single specific value such as a user-provided timestamp literal: Suppose further that the query executes under these conditions: Under those conditions, the comparison in the  WHERE  clause occurs in different ways for nonindexed and indexed lookups and leads to different results: Because the stored  ts  values are converted to the session time zone, it is possible for the query to return two timestamp values that are distinct as UTC values but equal in the session time zone: One 1731 •If there is no index or the optimizer cannot use it, comparisons occur in the session time zone. The optimizer performs a table scan in which it retrieves each  ts  column value, converts it from UTC to the session time zone, and compares it to the search value (also interpreted in the session time zone): •The query specifies a search value that is within the hour of entry into DST in the session time zone. •Unique UTC values stored in the  TIMESTAMP  column are not unique in the session time zone due to DST shifts. (The example shown earlier illustrates how this can occur.) •The session time zone is not UTC and has a DST shift. For example: SELECT ts FROM tstable WHERE ts = ’ literal ’; SET time_zone = ’MET’; mysql>  SELECT ts FROM tstable WHERE ts = ’2018-10-28 02:30:00’; +---------------------+ | ts | +---------------------+ | 2018-10-28 02:30:00 | | 2018-10-28 02:30:00 | +---------------------+ Note