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;
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 condition pushdown can be seen in the output of
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, condition pushdown cannot be used with the following query:
SELECT a,b FROM t1 WHERE a = 10;
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.)
Condition pushdown may also be employed when an indexed column
is compared with a constant using a
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 condition pushdown include the following:
patternmust be a string literal containing the pattern to be matched; for syntax, see Section 12.5.1, “String Comparison Functions and Operators”.
columnIS [NOT] NULL
Each item in the
value_listmust be a constant, literal value.
constant2must 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
variable. For example, in a
use these lines:
At runtime, disable condition pushdown like this:
Limitations. Engine condition pushdown is subject to the following limitations:
Condition pushdown is supported only by the
Prior to NDB 8.0.18, columns could be compared with constants or expressions which evaluate to constant values only. In NDB 8.0.18 and later, 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.
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
EXPLAINoutput to determine which conditions are actually pushed down. See Section 8.8.3, “Extended EXPLAIN Output Format”.
Previously, condition pushdown was limited to terms referring to column values from the same table to which the condition was being pushed. Beginning with NDB 8.0.16, 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.
Join algorithms cannot be combined with referring columns from previous tables in the following two situations:
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.
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.