Engine-defined New Table/Field/Index Attributes


  1. API
  2. SQL
  3. See Also

In MariaDB, a storage engine can allow the user to specify additional attributes per index, field, or table. The engine needs to declare what attributes it introduces.


There are three new members in the handlerton structure, they can be set in the engine's initialization function as follows:

example_hton->table_options= example_table_option_array;
example_hton->field_options= example_field_option_array;
example_hton->index_options= example_index_option_array;

The arrays are declared statically, as in the following example:

  "default value of the VAROPT table option", NULL, NULL, 5, 0, 100, 0);

struct ha_table_option_struct
  char *strparam;
  ulonglong ullparam;
  uint enumparam;
  bool boolparam;
  ulonglong varparam;

ha_create_table_option example_table_option_list[]=
  HA_TOPTION_NUMBER("NUMBER", ullparam, UINT_MAX32, 0, UINT_MAX32, 10),
  HA_TOPTION_STRING("STR", strparam),
  HA_TOPTION_ENUM("ONE_OR_TWO", enumparam, "one,two", 0),
  HA_TOPTION_BOOL("YESNO", boolparam, 1),
  HA_TOPTION_SYSVAR("VAROPT", varopt, varparam),

The engine declares a structure ha_table_option_struct that will hold values of these new attributes.

And it describes these attributes to MySQL by creating an array of HA_TOPTION_* macros. Note a detail: these macros expect a structure called ha_table_option_struct , if the structure is called differently, a #define will be needed.

There are five supported kinds of attributes:

macro nameattribure value typecorresponding C typeadditional parameters of a macro
HA_TOPTION_NUMBERan integer numberunsigned long longa default value, minimal allowed value, maximal allowed value, a factor, that any allowed should be a multiple of.
HA_TOPTION_STRINGa stringchar *none. The default value is a null pointer.
HA_TOPTION_ENUMone value from a list of allowed valuesunsigned inta string with a comma-separated list of allowed values, and a default value as a number, starting from 0.
HA_TOPTION_BOOLa booleanboola default value
HA_TOPTION_SYSVARdefined by the system variabledefined by the system variablesystem variable name

Do not use enum for your HA_TOPTION_ENUM C structure members, the size of the enum depends on the compiler, and even on the compilation options, and the plugin API uses only types with known storage sizes.

In all macros the first two parameters are name of the attribute as should be used in SQL in the CREATE TABLE statement, and the name of the corresponding member of the ha_table_option_struct structure.

The HA_TOPTION_SYSVAR stands aside a bit. It does not specify the attribute type or the default value, instead it binds the attribute to a system variable. The attribute type and the range of allowed values will be the same as of the corresponding system variable. The attribute default value will be the current value of its system variable. And unlike other attribute types that are only stored in the .frm file if explicitly set in the CREATE TABLE statement, the HA_TOPTION_SYSVAR attributes are always stored. If the system variable value is changed, it will not affect existing tables. Note that for this very reason, if a table was created in the old version of a storage engine, and a new version has introduced a HA_TOPTION_SYSVAR attribute, the attribute value in the old tables will be the default value of the system variable, not its current value.

The array ends with a HA_TOPTION_END macro.

Field and index (key) attributes are declared similarly using HA_FOPTION_* and HA_IOPTION_* macros.

When in a CREATE TABLE statement, the ::create() handler method is called, the table attributes are available in the table_arg->s->option_struct, field attributes - in the option_struct member of the individual fields (objects of the Field class), index attributes - in the option_struct member of the individual keys (objects of the KEY class).

Additionally, they are available in most other handler methods: the attributes are stored in the .frm file and on every open MySQL makes them available to the engine by filling the corresponding option_struct members of the table, fields, and keys.

The ALTER TABLE needs a special support from the engine. MySQL compares old and new table definitions to decide whether it needs to rebuild the table or not. As the semantics of the engine declared attributes is unknown, MySQL cannot make this decision by analyzing attribute values - this is delegated to the engine. The HA_CREATE_INFO structure has three new members:

ha_table_option_struct *option_struct;           ///< structure with parsed table options
ha_field_option_struct **fields_option_struct;   ///< array of field option structures
ha_index_option_struct **indexes_option_struct;  ///< array of index option structures

The engine (in the ::check_if_incompatible_data() method) is responsible for comparing new values of the attributes from the HA_CREATE_INFO structure with the old values from the table and returning COMPATIBLE_DATA_NO if they were changed in such a way that requires the table to be rebuild.

The example of declaring the attributes and comparing the values for the ALTER TABLE can be found in the EXAMPLE engine.


The engine declared attributes can be specified per field, index, or table in the CREATE TABLE or ALTER TABLE. The syntax is the conventional:

  field ... [attribute=value [attribute=value ...]],
  index ... [attribute=value [attribute=value ...]],
) ...  [attribute=value [attribute=value ...]]

All values must be specified as literals, not expressions. The value of a boolean option may be specified as one of YES, NO, ON, OFF, 1, or 0. A string value may be specified either quoted or not, as an identifier (if it is a valid identifier, of course). Compare with the old behavior:


where the value of the ENGINE attribute is specified not quoted, while the value of the CONNECTION is quoted.

When an attribute is set, it will be stored with the table definition and shown in the SHOW CREATE TABLE; . To remove an attribute from a table definition use ALTER TABLE to set its value to a DEFAULT .

The values of unknown attributes or attributes with the illegal values cause an error by default. But with ALTER TABLE one can change the storage engine and some previously valid attributes may become unknown — to the new engine. They are not removed automatically, though, because the table might be altered back to the first engine, and these attributes will be valid again. Still SHOW CREATE TABLE will comment these unknown attributes out in the output, otherwise they would make a generated CREATE TABLE statement invalid.

With the IGNORE_BAD_TABLE_OPTIONS sql mode this behavior changes. Unknown attributes do not cause an error, they only result in a warning. And SHOW CREATE TABLE will not comment them out. This mode is implicitly enabled in the replication slave thread.

See Also


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