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Lateral Derived Optimization

MariaDB supports the Lateral Derived optimization, also referred to as "Split Grouping Optimization" or "Split Materialized Optimization" in some sources.

Description

The optimization's use case is

  • The query uses a derived table (or a VIEW, or a non-recursive CTE)

  • The derived table/View/CTE has a GROUP BY operation as its top-level operation

  • The query only needs data from a few GROUP BY groups

An example of this: consider a VIEW that computes totals for each customer in October:

CREATE VIEW OCT_TOTALS AS
SELECT
  customer_id,
  SUM(amount) AS TOTAL_AMT
FROM orders
WHERE
  order_date BETWEEN '2017-10-01' AND '2017-10-31'
GROUP BY
  customer_id;

And a query that does a join with the customer table to get October totals for "Customer#1" and Customer#2:

SELECT *
FROM
  customer, OCT_TOTALS
WHERE
  customer.customer_id=OCT_TOTALS.customer_id AND
  customer.customer_name IN ('Customer#1', 'Customer#2')

Before Lateral Derived optimization, MariaDB would execute the query as follows:

  1. Materialize the view OCT_TOTALS. This essentially computes OCT_TOTALS for all customers.

  2. Join it with table customer.

The EXPLAIN would look like so:

+------+-------------+------------+-------+---------------+-----------+---------+---------------------------+-------+--------------------------+
| id   | select_type | table      | type  | possible_keys | key       | key_len | ref                       | rows  | Extra                    |
+------+-------------+------------+-------+---------------+-----------+---------+---------------------------+-------+--------------------------+
|    1 | PRIMARY     | customer   | range | PRIMARY,name  | name      | 103     | NULL                      | 2     | Using where; Using index |
|    1 | PRIMARY     | <derived2> | ref   | key0          | key0      | 4       | test.customer.customer_id | 36    |                          |
|    2 | DERIVED     | orders     | index | NULL          | o_cust_id | 4       | NULL                      | 36738 | Using where              |
+------+-------------+------------+-------+---------------+-----------+---------+---------------------------+-------+--------------------------+

It is obvious that Step #1 is very inefficient: we compute totals for all customers in the database, while we will only need them for two customers. (If there are 1000 customers, we are doing 500x more work than needed here)

Lateral Derived optimization addresses this case. It turns the computation of OCT_TOTALS into what SQL Standard refers to as "LATERAL subquery": a subquery that may have dependencies on the outside tables. This allows pushing the equality customer.customer_id=OCT_TOTALS.customer_id down into the derived table/view, where it can be used to limit the computation to compute totals only for the customer of interest.

The query plan will look as follows:

  1. Scan table customer and find customer_id for Customer#1 and Customer#2.

  2. For each customer_id, compute the October totals, for this specific customer.

The EXPLAIN output will look like so:

+------+-----------------+------------+-------+---------------+-----------+---------+---------------------------+------+--------------------------+
| id   | select_type     | table      | type  | possible_keys | key       | key_len | ref                       | rows | Extra                    |
+------+-----------------+------------+-------+---------------+-----------+---------+---------------------------+------+--------------------------+
|    1 | PRIMARY         | customer   | range | PRIMARY,name  | name      | 103     | NULL                      | 2    | Using where; Using index |
|    1 | PRIMARY         | <derived2> | ref   | key0          | key0      | 4       | test.customer.customer_id | 2    |                          |
|    2 | LATERAL DERIVED | orders     | ref   | o_cust_id     | o_cust_id | 4       | test.customer.customer_id | 1    | Using where              |
+------+-----------------+------------+-------+---------------+-----------+---------+---------------------------+------+--------------------------+

Note the line with id=2: select_type is LATERAL DERIVED. And table customer uses ref access referring to customer.customer_id, which is normally not allowed for derived tables.

In EXPLAIN FORMAT=JSON output, the optimization is shown like so:

...
        "table": {
          "table_name": "<derived2>",
          "access_type": "ref",
...
          "materialized": {
            "lateral": 1,

Note the "lateral": 1 member.

Controlling the Optimization

Lateral Derived is enabled by default. The optimizer will make a cost-based decision whether the optimization should be used.

If you need to disable the optimization, it has an optimizer_switch flag. It can be disabled like so:

SET optimizer_switch='split_materialized=off'

From MariaDB 12.1, it is possible to enable or disable the optimization with an optimizer hint, SPLIT_MATERLIZED or NO_SPLIT_MATERIALIZED.

For example, by default, this table and query makes use of the optimization:

CREATE TABLE t1 ( n1 INT(10) NOT NULL, n2 INT(10) NOT NULL, c1 CHAR(1) NOT NULL, KEY c1 (c1), KEY n1_c1_n2 (n1,c1,n2) ) ENGINE=innodb CHARSET=latin1;

INSERT INTO t1 VALUES (0, 2, 'a'), (1, 3, 'a');

INSERT INTO t1 SELECT seq+1,seq+2,'c' FROM seq_1_to_1000;

ANALYZE TABLE t1;

EXPLAIN SELECT t1.n1 FROM t1, (SELECT n1, n2 FROM t1 WHERE c1 = 'a' GROUP BY n1) AS t WHERE t.n1 = t1.n1 AND t.n2 = t1.n2 AND c1 = 'a' GROUP BY n1\G

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
         type: ref
possible_keys: c1,n1_c1_n2
          key: c1
      key_len: 1
          ref: const
         rows: 2
        Extra: Using index condition; Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
         type: ref
possible_keys: key0
          key: key0
      key_len: 8
          ref: test.t1.n1,test.t1.n2
         rows: 1
        Extra: 
*************************** 3. row ***************************
           id: 2
  select_type: LATERAL DERIVED
        table: t1
         type: ref
possible_keys: c1,n1_c1_n2
          key: n1_c1_n2
      key_len: 4
          ref: test.t1.n1
         rows: 1
        Extra: Using where; Using index

The optimization can be disabled as follows:

EXPLAIN SELECT /*+ NO_SPLIT_MATERIALIZED(t) */ t1.n1 FROM t1, (SELECT n1, n2 FROM t1 WHERE c1 = 'a' GROUP BY n1) AS t WHERE t.n1 = t1.n1 AND t.n2 = t1.n2 AND c1 = 'a' GROUP BY n1\G

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t1
         type: ref
possible_keys: c1,n1_c1_n2
          key: c1
      key_len: 1
          ref: const
         rows: 2
        Extra: Using index condition; Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
         type: ref
possible_keys: key0
          key: key0
      key_len: 8
          ref: test.t1.n1,test.t1.n2
         rows: 1
        Extra: 
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: t1
         type: ref
possible_keys: c1
          key: c1
      key_len: 1
          ref: const
         rows: 2
        Extra: Using index condition; Using where; Using temporary; Using filesort

References

This page is licensed: CC BY-SA / Gnu FDL

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