Question

I’m working with session variables and I would want to create a view which does the following:

if (myVar=1)
  select * from my_table where X
else
  select * from my_table where Y

Where X and Y are different clauses.

Is it possible?

Can this be implemented with a regular select statement (or I need stored procedures)?

Answer 1

Try something like this:

SELECT 'A'
FROM tableA
WHERE current_setting(setting_name) = 'setting A'
UNION ALL
SELECT 'B'
FROM tableB
WHERE current_setting(setting_name) = 'setting B'

Details on postgresql session variables here.

UPD It will give the results of one of the SELECT. If current_setting(setting_name) equals to 'setting A' the first query will return the results, but the second wont.

For your example the query will look like:

SELECT 'A'
FROM tableA
WHERE myVar = 1
UNION ALL
SELECT 'B'
FROM tableB
WHERE myVar != 1

UPD Checked: postgres executes only one of the queries. EXPLAIN ANALYZE shows that the second query was planned but marked as (never executes).