10.2.1.5 Engine Condition Pushdown Optimization

This optimization improves the efficiency of direct comparisons between a nonindexed column and a constant. In such cases, the condition is “pushed down” to the storage engine for evaluation. This optimization can be used only by the NDB storage engine.

For NDB Cluster, this optimization can eliminate the need to send nonmatching rows over the network between the cluster's data nodes and the MySQL server that issued the query, and can speed up queries where it is used by a factor of 5 to 10 times over cases where condition pushdown could be but is not used.

Suppose that an NDB Cluster table is defined as follows:

CREATE TABLE t1 (
    a INT,
    b INT,
    KEY(a)
) ENGINE=NDB;

Engine condition pushdown can be used with queries such as the one shown here, which includes a comparison between a nonindexed column and a constant:

SELECT a, b FROM t1 WHERE b = 10;

The use of engine condition pushdown can be seen in the output of EXPLAIN:

mysql> EXPLAIN SELECT a, b FROM t1 WHERE b = 10\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 10
        Extra: Using where with pushed condition

However, engine condition pushdown cannot be used with the following query:

SELECT a,b FROM t1 WHERE a = 10;

Engine condition pushdown is not applicable here because an index exists on column a. (An index access method would be more efficient and so would be chosen in preference to condition pushdown.)

Engine condition pushdown may also be employed when an indexed column is compared with a constant using a > or < operator:

mysql> EXPLAIN SELECT a, b FROM t1 WHERE a < 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t1
         type: range
possible_keys: a
          key: a
      key_len: 5
          ref: NULL
         rows: 2
        Extra: Using where with pushed condition

Other supported comparisons for engine condition pushdown include the following:

• column [NOT] LIKE pattern

  pattern must be a string literal containing the pattern to be matched; for syntax, see Section 14.8.1, “String Comparison Functions and Operators”.

• column IS [NOT] NULL

• column IN (value_list)

  Each item in the value_list must be a constant, literal value.

• column BETWEEN constant1 AND constant2

  constant1 and constant2 must each be a constant, literal value.

In all of the cases in the preceding list, it is possible for the condition to be converted into the form of one or more direct comparisons between a column and a constant.

Engine condition pushdown is enabled by default. To disable it at server startup, set the optimizer_switch system variable's engine_condition_pushdown flag to off. For example, in a my.cnf file, use these lines:

[mysqld]
optimizer_switch=engine_condition_pushdown=off

At runtime, disable condition pushdown like this:

SET optimizer_switch='engine_condition_pushdown=off';

Limitations.  Engine condition pushdown is subject to the following limitations:

• Engine condition pushdown is supported only by the NDB storage engine.

• In NDB 8.4, columns can be compared with one another as long as they are of exactly the same type, including the same signedness, length, character set, precision, and scale, where these are applicable.

• Columns used in comparisons cannot be of any of the BLOB or TEXT types. This exclusion extends to JSON, BIT, and ENUM columns as well.

• A string value to be compared with a column must use the same collation as the column.

• Joins are not directly supported; conditions involving multiple tables are pushed separately where possible. Use extended EXPLAIN output to determine which conditions are actually pushed down. See Section 10.8.3, “Extended EXPLAIN Output Format”.

Previously, engine condition pushdown was limited to terms referring to column values from the same table to which the condition was being pushed. In NDB 8.4, column values from tables earlier in the query plan can also be referred to from pushed conditions. This reduces the number of rows which must be handled by the SQL node during join processing. Filtering can be also performed in parallel in the LDM threads, rather than in a single mysqld process. This has the potential to improve performance of queries by a significant margin.

NDB can push an outer join using a scan if there are no unpushable conditions on any table used in the same join nest, or on any table in join nests above it on which it depends. This is also true for a semijoin, provided the optimization strategy employed is firstMatch (see Optimizing IN and EXISTS Subquery Predicates with Semijoin Transformations).

Join algorithms cannot be combined with referring columns from previous tables in the following two situations:

1. When any of the referred previous tables are in a join buffer. In this case, each row retrieved from the scan-filtered table is matched against every row in the buffer. This means that there is no single specific row from which column values can be fetched from when generating the scan filter.

2. When the column originates from a child operation in a pushed join. This is because rows referenced from ancestor operations in the join have not yet been retrieved when the scan filter is generated.

Columns from ancestor tables in a join can be pushed down, provided that they meet the requirements listed previously. An example of such a query, using the table t1 created previously, is shown here:

mysql> EXPLAIN 
    ->   SELECT * FROM t1 AS x 
    ->   LEFT JOIN t1 AS y 
    ->   ON x.a=0 AND y.b>=3\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: x
   partitions: p0,p1
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 4
     filtered: 100.00
        Extra: NULL
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: y
   partitions: p0,p1
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 4
     filtered: 100.00
        Extra: Using where; Using pushed condition (`test`.`y`.`b` >= 3); Using join buffer (hash join)
2 rows in set, 2 warnings (0.00 sec)