PostgreSQL 8.2.6 Documentation  

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This section describes functions that possibly return more than one row. Currently the only functions in this class are series generating functions, as detailed in Table 939 .
Table 939. Series Generating Functions
Function  Argument Type  Return Type  Description 

generate_series
(
start
,
stop
)

int or bigint  setof int or setof bigint (same as argument type)  Generate a series of values, from start to stop with a step size of one 
generate_series
(
start
,
stop
,
step
)

int or bigint  setof int or setof bigint (same as argument type)  Generate a series of values, from start to stop with a step size of step 
When step is positive, zero rows are returned if start is greater than stop . Conversely, when step is negative, zero rows are returned if start is less than stop . Zero rows are also returned for NULL inputs. It is an error for step to be zero. Some examples follow:
select * from generate_series(2,4); generate_series  2 3 4 (3 rows) select * from generate_series(5,1,2); generate_series  5 3 1 (3 rows) select * from generate_series(4,3); generate_series  (0 rows) select current_date + s.a as dates from generate_series(0,14,7) as s(a); dates  20040205 20040212 20040219 (3 rows)