After a query has produced an output table (after the select list has been processed) it can optionally be sorted. If sorting is not chosen, the rows will be returned in an unspecified order. The actual order in that case will depend on the scan and join plan types and the order on disk, but it must not be relied on. A particular output ordering can only be guaranteed if the sort step is explicitly chosen.
   The ORDER BY clause specifies the sort order:
SELECTselect_listFROMtable_expressionORDER BYsort_expression1[ASC | DESC] [NULLS { FIRST | LAST }] [,sort_expression2[ASC | DESC] [NULLS { FIRST | LAST }] ...]
The sort expression(s) can be any expression that would be valid in the query's select list. An example is:
SELECT a, b FROM table1 ORDER BY a + b, c;
   When more than one expression is specified,
   the later values are used to sort rows that are equal according to the
   earlier values.  Each expression can be followed by an optional
   ASC or DESC keyword to set the sort direction to
   ascending or descending.  ASC order is the default.
   Ascending order puts smaller values first, where
   “smaller” is defined in terms of the
   < operator.  Similarly, descending order is
   determined with the > operator.
    [5]
  
   The NULLS FIRST and NULLS LAST options can be
   used to determine whether nulls appear before or after non-null values
   in the sort ordering.  By default, null values sort as if larger than any
   non-null value; that is, NULLS FIRST is the default for
   DESC order, and NULLS LAST otherwise.
  
   Note that the ordering options are considered independently for each
   sort column.  For example ORDER BY x, y DESC means
   ORDER BY x ASC, y DESC, which is not the same as
   ORDER BY x DESC, y DESC.
  
   A sort_expression can also be the column label or number
   of an output column, as in:
SELECT a + b AS sum, c FROM table1 ORDER BY sum; SELECT a, max(b) FROM table1 GROUP BY a ORDER BY 1;
both of which sort by the first output column. Note that an output column name has to stand alone, that is, it cannot be used in an expression — for example, this is not correct:
SELECT a + b AS sum, c FROM table1 ORDER BY sum + c; -- wrong
   This restriction is made to reduce ambiguity.  There is still
   ambiguity if an ORDER BY item is a simple name that
   could match either an output column name or a column from the table
   expression.  The output column is used in such cases.  This would
   only cause confusion if you use AS to rename an output
   column to match some other table column's name.
  
   ORDER BY can be applied to the result of a
   UNION, INTERSECT, or EXCEPT
   combination, but in this case it is only permitted to sort by
   output column names or numbers, not by expressions.
  
[5]       Actually, PostgreSQL uses the default B-tree
      operator class for the expression's data type to determine the sort
      ordering for ASC and DESC.  Conventionally,
      data types will be set up so that the < and
      > operators correspond to this sort ordering,
      but a user-defined data type's designer could choose to do something
      different.