MyRocks column families¶
MyRocks stores all data in a single server instance as a collection of key-value pairs within the log structured merge tree data structure. This is a flat data structure that requires that keys be unique throughout the whole data structure. MyRocks incorporates table IDs and index IDs into the keys.
Each key-value pair belongs to a column family. It is a data structure similar in concept to tablespaces. Each column family has distinct attributes, such as block size, compression, sort order, and MemTable. Utilizing these attributes, MyRocks effectively uses column families to store indexes.
On system initialization, MyRocks creates two column families. The
__system__ column family is reserved by MyRocks; no user created tables or
indexes belong to this column family. The default column family is the
location for the indexes created by the user when you a column family is not
explicitly specified.
To be able to apply a custom block size, compression, or sort order you need to
create an index in its own column family using the COMMENT clause.
The following example demonstrates how to place the PRIMARY KEY into the
cf1 column family and the index kb — into the cf2 column family.
CREATE TABLE t1 (a INT, b INT,
PRIMARY KEY(a) COMMENT 'cfname=cf1',
KEY kb(b) COMMENT 'cfname=cf2')
ENGINE=ROCKSDB;
The column family name is specified as the value of the cfname attribute at
the beginning of the COMMENT clause. The name is case sensitive and may not
contain leading or trailing whitespace characters.
The COMMENT clause may contain other information following the semicolon
character (;) after the column family name: ‘cfname=foo; special column
family’. If the column family cannot be created, MyRocks uses the default
column family.
Warning
The cfname attribute must be all lowercase. Place the equals sign (=) in front of the column family name without any whitespace on both sides of it.
COMMENT 'cfname=Foo; Creating the Foo family name'
See also
Using COMMENT to Specify Column Family Names with Multiple Table Partitions
https://github.com/facebook/mysql-5.6/wiki/Column-Families-on-Partitioned-Tables.
Controlling the number of column families to reduce memory consumption¶
Each column family has its own MemTable. It is an in-memory data structure where data are written to before they are flushed to SST files. The queries also use MemTables first. To reduce the overall memory consumption, the number of active column families should stay low.
With the option |opt.no-create-column-family| set to true, the COMMENT
clause will not treat cfname as a special token; it will not be possible to
create column families using the COMMENT clause.
Column family options¶
On startup, the server applies the |opt.default-cf-options| option to all
existing column families. You may use the |opt.override-cf-options| option to
override the value of any attribute of a chosen column family.
Note that the options |opt.dcfo| and |opt.ocfo| are read-only at runtime.
At runtime, use the the |opt.update-cf-options| option to update some column
family attributes.
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