Most DBMSs provide some non-standard way to override the optimizer if you judge that its estimates or formulas are wrong. Commonly the mechanism is a hint, which often looks like a comment or parenthetical clause in the SQL statement but in fact is a direction to the optimizer. Here's an example from Oracle:
SELECT /*+ INDEX(Table1 Index1) */ column1, column2 FROM Table1 WHERE column1 >55
And here's one from Sybase:
SELECT column1, column2 FROM Table1 (INDEX Index1 PREFETCH 16) WHERE column1 > 55
Hints are important enough to warrant mention, but there is no such thing as a portable hint; every DBMS (except IBM) has its own optimizer-specific hint set. We'll therefore limit ourselves to listing the most common hints, without getting into syntax.
DBA note: Administrators can influence some DBMS's plans more generally with a manual override of the statistics, for example, by declaring that the selectivity of an index is 80% rather than 40%.