17.10 - All Recursive View Definitions Must Be Linear - Advanced SQL Engine - Teradata Database

Teradata Vantage™ - SQL Data Definition Language Detailed Topics

Product
Advanced SQL Engine
Teradata Database
Release Number
17.10
Release Date
July 2021
Content Type
Programming Reference
Publication ID
B035-1184-171K
Language
English (United States)

SQL does not support non-linear recursion.

When recursion is linear, invoking a recursive view produces at most 1 direct invocation of that recursive view. To enforce linearity, SQL restricts table references to a single occurrence in the recursive view definition, which can be specified either in the FROM clause of the recursive statement or in a subquery in the recursive statement, but not in both.

A recursion is said to be linear if it obeys the following rules.

  • For every joined table in the recursive view definition, the recursively-defined table is referenced only once.
  • The recursively-defined table is not referenced in both the FROM clause and in a subquery of the same query specification.
  • The recursively-defined table is not referenced more than once in the FROM clause of the query specification.

If any of these conditions is not met, then the recursion is non-linear.

The following create text is an example of a non-valid attempt to create a recursive view definition. The definition is not valid because the recursive view Fibonacci is referenced more than once (using the column alias names p and pp, respectively) in the FROM clause of the recursively-defined relation in the view definition. As a result, it violates the rule stated in the third bullet.

     CREATE RECURSIVE VIEW fibonacci (n, f, mycount) AS
       SELECT a, b, 0 AS mycount
       FROM t
       WHERE (a=0
       AND    b=0)
       OR    (a=1
       AND    b=1)
     UNION ALL
       SELECT n+1, p.f + pp.f, p.mycount + 1
       FROM fibonacci AS p, fibonacci AS pp
       WHERE (p.n - 1) = pp.n
       AND   p.mycount <= 100;

The following similarly conceived query that uses derived tables has the same problem.

     CREATE RECURSIVE VIEW fibonacci (n, f, mycount) AS
       SELECT  n, f, mycount
       FROM (SELECT 0,0,0) AS a (n,f,mycount)
     UNION ALL
       SELECT n, f, mycount
       FROM (SELECT 1,1,0) AS b (n, f, mycount)
     UNION ALL
       SELECT n+1, p.f + pp.f, p.mycount + 1
       FROM fibonacci AS p, fibonacci AS pp
       WHERE (p.n - 1) = pp.n
       AND   p.mycount <= 100;