Defines (if distinct is true) or removes (if distinct is false) a DISTINCT clause
for a SELECT statement.
If distinct is true, then the ID of an expression can be specified as the expr_id argument:
if not 0, this is the expression used to apply the DISTINCT clause on (the resuting SQL
will then usually be "... DISTINCT ON <expression>...").
Defines (if distinct is true) or removes (if distinct is false) a DISTINCT clause for a SELECT statement.
If distinct is true, then the ID of an expression can be specified as the expr_id argument: if not 0, this is the expression used to apply the DISTINCT clause on (the resuting SQL will then usually be "... DISTINCT ON <expression>...").