16.20 - Building a Recursive View - Teradata Database - Teradata Vantage NewSQL Engine

Teradata Vantage™ SQL Data Definition Language Detailed Topics

Product
Teradata Database
Teradata Vantage NewSQL Engine
Release Number
16.20
Release Date
March 2019
Content Type
Programming Reference
Publication ID
B035-1184-162K
Language
English (United States)
The basic definition of a recursive view consists of these component classes.
  • A non-recursive, or seed query class.
  • A recursive query class.
  • A UNION ALL set operator to connect the query components.
  • A terminal condition to prevent infinite recursion.

A recursive view definition can contain multiple seed and recursive statements.

Normally, you should specify the terminal condition explicitly in the WHERE clause of the recursive statement, but such an explicit condition is not mandatory because recursive queries are implicitly limited by limits on user spool space and system disk space.

Consider the following basic BNF (BNF, or Backus-Naur Form, is a metalanguage used to define computer languages) for the CREATE RECURSIVE VIEW statement.

     CREATE RECURSIVE VIEW <view_name> [(<column_name_list>)] AS (
     <query_expression_1> UNION ALL <query_expression_2>

where:

Syntax element … Specifies …
<view_name> the name by which the view is referenced.
<column_name_list> a set of base table column names and expressions from which the view referenced by <view_name> is to be constructed.
<query_expression_1> a seed statement.
<query_expression_2> a recursive statement.

Angle bracket characters (<>) indicate an identifier or complex syntax element and straight bracket characters ([]) indicate an optional syntax component.

The following example is based on the Flights table defined in The Concept of Recursion.

    CREATE RECURSIVE VIEW reachable_from (source,destination,depth) AS (
      SELECT root.source, root.destination, 0 AS depth
      FROM flights AS root
      WHERE root.source = 'Paris'
    UNION ALL
      SELECT in1.source, out1.destination, in1.depth + 1
      FROM reachable_from AS in1, flights AS out1
      WHERE in1.destination = out1.source
      AND   in1.depth <= 100);

How does this example related to the BNF for CREATE RECURSIVE VIEW? The units of the BNF correspond to this example recursive view definition as follows.

BNF element … Corresponds to this example query element …
view_name reachable_from
column_name_list source, destination, depth
query_expression_1
SELECT root.source, root.destination, 0 AS depth
FROM flights AS root
WHERE root.source = 'Paris'

This is the seed statement (see Seed Statement Component of the View Definition) for the recursive view definition. The statement does not reference any recursive relations at any point in its definition.

query_expression_2
SELECT in1.source,out1.destination,in1.depth + 1
FROM reachable_from AS in1, flights AS out1
WHERE in1.destination = out1.source
AND   in1.depth <=100

This is the recursive statement (see Recursive Statement Component of the View Definition) for the recursive view definition. The recursion occurs because the view name reachable_from is referenced in the FROM clause of this statement.