GaussDB(DWS) WITH Expressions

SELECT in WITH

This section describes the usage of SELECT in a WITH clause.

Syntax

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[WITH [RECURSIVE] with_query [, ...] ] SELECT ...

The syntax of with_query is as follows:

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with_query_name [ ( column_name [, ...] ) ]
    AS [ [ NOT ] MATERIALIZED ] ( {select | values | insert | update | delete} )

• If you use MATERIALIZED, the subquery runs once and its result set is saved. If you use NOT MATERIALIZED, the subquery is replaced with its reference in the main query.
• The SQL statement specified by the AS statement of a CTE must be a statement that can return query results. It can be a common SELECT query statement or other data modification statements such as INSERT, UPDATE, DELETE, and VALUES. When using a data modification statement, you need to use the RETURNING clause to return tuples. Example:

  1	WITH s AS (INSERT INTO t VALUES(1) RETURNING a) SELECT * FROM s;

• A WITH expression indicates the CTE definition in a SQL statement block. Multiple CTEs can be defined at the same time. You can specify column names for each CTE or use the aliases of the columns in the query output. Example:

  1	WITH s1(a, b) AS (SELECT x, y FROM t1), s2 AS (SELECT x, y FROM t2) SELECT * FROM s1 JOIN s2 ON s1.a=s2.x;

  This statement defines two CTEs: s1 and s2. s1 specifies the column names a and b, and s2 does not specify the column names. Therefore, the column names are the output column names x and y.

• Each CTE can be referenced zero, one, or more times in the main query.
• CTEs with the same name cannot exist in the same statement block. If CTEs with the same name exist in different statement blocks, the CTE in the nearest statement block is referenced.
• An SQL statement may contain multiple SQL statement blocks. Each statement block can contain a WITH expression. The CTE in each WITH expression can be referenced in the current statement block, subsequent CTEs of the current statement block, and sub-layer statement blocks, however, it cannot be referenced in the parent statement block. The definition of each CTE is also a statement block. Therefore, a WITH expression can also be defined in the statement block.

The purpose of SELECT in WITH is to break down complex queries into simple parts. Example:

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    WITH regional_sales AS (
         SELECT region, SUM(amount) AS total_sales
         FROM orders
         GROUP BY region
     ), top_regions AS (
         SELECT region
         FROM regional_sales
         WHERE total_sales > (SELECT SUM(total_sales)/10 FROM regional_sales)
     )
     SELECT region,
            product,
            SUM(quantity) AS product_units,
            SUM(amount) AS product_sales
     FROM orders
     WHERE region IN (SELECT region FROM top_regions)
     GROUP BY region, product;

The WITH clause defines two auxiliary statements: regional_sales and top_regions. The output of regional_sales is used in top_regions, and the output of top_regions is used in the main SELECT query. This example can be written without WITH. In that case, it must be written with a two-layer nested sub-SELECT statement, making the query longer and difficult to maintain.

Recursive WITH Query

By declaring the keyword RECURSIVE, a WITH query can reference its own output.

The common form of a recursive WITH query is as follows:

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non_recursive_term UNION [ALL] recursive_term

UNION performs deduplication when merging sets, while UNION ALLL directly merges result sets without deduplication. Only recursive items can contain references to the output of the query itself.

When using recursive WITH, ensure that the recursive item of the query does not return a tuple. Otherwise, the query will loop infinitely.

The table tree is used to store information about all nodes in the following figure.

The table definition statement is as follows:

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CREATE TABLE tree(id INT, parentid INT);

The data in the table is as follows:

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INSERT INTO tree VALUES(1,0),(2,1),(3,1),(4,2),(5,2),(6,3),(7,3),(8,4),(9,4),(10,6),(11,6),(12,10);

SELECT * FROM tree;
 id | parentid
----+----------
  1 |        0
  2 |        1
  3 |        1
  4 |        2
  5 |        2
  6 |        3
  7 |        3
  8 |        4
  9 |        4
 10 |        6
 11 |        6
 12 |       10
(12 rows)

You can run the following WITH RECURSIVE statement to return the nodes and hierarchy information of the entire tree starting from node 1 at the top layer:

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WITH RECURSIVE nodeset AS
(
-- recursive initializing query
SELECT id, parentid, 1 AS level FROM tree
WHERE id = 1
UNION ALL
-- recursive join query
SELECT tree.id, tree.parentid, level + 1 FROM tree, nodeset 
WHERE tree.parentid = nodeset.id 
)
SELECT * FROM nodeset ORDER BY id;

In the preceding query, a typical WITH RECURSIVE expression contains the CTE of at least one recursive query. The CTE is defined as a UNION ALL set operation. The first branch is the recursive start query, and the second branch is the recursive join query, the first part is referenced for continuous recursive join. When this statement is executed, the recursive start query is executed once, and the join query is executed several times. The results are added to the start query result set until the results of some join queries are empty.

The command output is as follows:

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 id | parentid | level
----+----------+-------
  1 |        0 |     1
  2 |        1 |     2
  3 |        1 |     2
  4 |        2 |     3
  5 |        2 |     3
  6 |        3 |     3
  7 |        3 |     3
  8 |        4 |     4
  9 |        4 |     4
 10 |        6 |     4
 11 |        6 |     4
 12 |       10 |     5
(12 rows)

According to the returned result, the start query result contains the result set whose level is 1. The join query is executed for five times. The result sets whose levels are 2, 3, 4, and 5 are output for the first four times. During the fifth execution, there is no record whose parentid is the same as the output result set ID, that is, there is no redundant child node. Therefore, the query ends.

GaussDB(DWS) supports distributed execution of WITH RECURSIVE expressions. WITH RECURSIVE involves cyclic calculation. Therefore, GaussDB(DWS) introduces the max_recursive_times parameter to control the maximum number of cycles of WITH RECURSIVE. The default value is 200. If the number of cycles exceeds 200, an error is reported.

Data Modification Statements in WITH

Use the INSERT, UPDATE, and DELETE commands in the WITH clause. This allows the user to perform multiple different operations in the same query. The following is an example:

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WITH moved_tree AS (
     DELETE FROM tree
     WHERE parentid = 4 
     RETURNING * )
 INSERT INTO tree_log
 SELECT * FROM moved_tree;

The preceding query example actually moves rows from tree to tree_log. The DELETE command in the WITH clause deletes the specified rows from tree, returns their contents through the RETURNING clause, and then the main query reads the output and inserts it into tree_log.

To retrieve the modified content instead of the target table, the data modification statement in the WITH clause should include the RETURNING clause. This clause creates a temporary table that can be accessed by the rest of the query. If a data modification statement in the WITH statement lacks a RETURNING clause, it cannot form a temporary table and cannot be referenced in the remaining queries.

If the RECURSIVE keyword is declare, recursive self-reference is not allowed in data modification statements. In some cases, you can bypass this restriction by referencing the output of recursive the WITH statement. For example:

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WITH RECURSIVE included_parts(sub_part, part) AS (
     SELECT sub_part, part FROM parts WHERE part = 'our_product'
   UNION ALL
     SELECT p.sub_part, p.part
     FROM included_parts pr, parts p
     WHERE p.part = pr.sub_part
   ) 
DELETE FROM parts
   WHERE part IN (SELECT part FROM included_parts);

This query will remove all direct or indirect subparts of a product.

The substatements in the WITH clause are executed at the same time as the main query. Therefore, when using the data modification statement in a WITH statement, the actual update order is in an unpredictable manner. All statements are executed in the same snapshot, and the effect of the statements is invisible on the target table. This mitigates the unpredictability of the actual order of row updates and means that RETURNING data is the only way to convey changes between different WITH substatements and the main query.

In this example, the outer layer SELECT can return the data before the update.

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WITH t AS (
     UPDATE tree SET id = id + 1
     RETURNING * ) 
SELECT * FROM tree;

In this example, the external SELECT returns the updated data.

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WITH t AS (     
UPDATE tree SET id = id + 1
     RETURNING * )
SELECT * FROM t;

The same row cannot be updated twice in a single statement. Otherwise, the update effect will be unpredictable. If only one update takes effect, it is difficult (and sometimes impossible) to predict which one takes effect.