This article introduces Mysql Metadata

Mysql Metadata is anything that describes the database as opposed to being the contents of the database. Metadata is comprised of 2 parts:

  • Data Dictionary
  • System tables

1. Data Dictionary

dd Mysql Server incorporates a transactional data dictionary that stores information about database objects. The data dictionary schema stores dictionary data in transactional(InnoDB) tables. Data dictionary tables are located in the mysql database. Data dictionary tables are created in a single InnoDB tablespace named mysql.ibd, which resides in the MySql data directory.

Basic Data Dictionary Tables

  • catalogs catalog information
  • schemata information about schemata
  • tablespaces active tablespaces
  • tables tables in databases
  • columns columns in tables
  • indexes information about table indexes

information_schema is now implemented as views over dictionary tables, requires no extra disc accesses, no creation of temporary tables.

1.1. 1.1 Meta Data Lock

  • MDL_context context of the owner of metadata locks
  • MDL_ticket a granted meta data lock
class THD {
    MDL_context mdl_context
}

class Table_ref  {
    char *db
    char *table_name
    LEX_CSTRING target_tablespace_name
    table_map m_map
    table_map sj_inner_tables
    Table_ref *natural_join
    List<Natural_join_column> *join_columns
    TABLE *table

    ST_SCHEMA_TABLE *schema_table
    Query_block *schema_query_block

    Query_block *query_block


    MDL_request mdl_request
}

class MDL_context {
    MDL_context_owner *m_owner
    enum_mdl_type type
    MDL_ticket_store m_ticket_store

    acquire_locks(*mdl_request)

}

class  MDL_request {
    MDL_request *next_in_list
    MDL_request **prev_in_list
    MDL_ticket *ticket
    MDL_key key
}

class MDL_ticket  {
    MDL_context *m_ctx
    MDL_lock *m_lock
}

class MDL_key

class MDL_map {
    LF_HASH m_locks
    MDL_lock *m_global_lock
    MDL_lock *m_commit_lock
    MDL_lock *m_acl_cache_lock
    MDL_lock *m_backup_lock
}

class MDL_lock {
    {static} MDL_map mdl_locks
}

Table_ref *-- MDL_request
MDL_request *-- MDL_key
MDL_request -> MDL_context: require
THD *-- MDL_context
MDL_context *-- MDL_ticket
MDL_request *-- MDL_ticket
MDL_map o-- MDL_lock
MDL_ticket *- MDL_lock

1.2. Table_cache

Table_cache cache for opened tables Table_cache_manager container class for all the Table_cache instances in the system Table_cache_element Element that represents the table in the specific table cache

class Table_cache_manager {
    Table_cache m_table_cache[MAX_TABLE_CACHES]
}

class Table_cache {
    TABLE *m_unused_tables
    map<string, Table_cache_element > m_cache
}

class Table_cache_element {
    TABLE_list used_tables
    TABLE_list free_tables_slim
    TABLE_SHARE *share
}

struct Table {
    THD *in_use
    Field **field
    TABLE_SHARE *s
    handler *file
    TABLE *next
    TABLE *prev
    TABLE *cache_next
    TABLE **cache_prev
    partition_info *part_info
}


Table_cache_manager o-- Table_cache
Table_cache o-- Table_cache_element
Table_cache_element o- Table

1.3. Data Dictionary Cache

Dictionary_client provides a unified interface to accessing dictionary objects. The main task of the client is to access a shared cache to retrive dictionary objects. The shared cache in its turn, will access dictionary tables if there is a cache miss.

class Dictionary_client {
    std::vector<Entity_object *> m_uncached_objects
  Object_registry m_registry_committed
  Object_registry m_registry_uncommitted
  Object_registry m_registry_dropped
  THD *m_thd                      
  Auto_releaser m_default_releaser
  Auto_releaser *m_current_releaser

  bool acquire(&key, **object,...)
  void acquire_uncommitted(&key, **object)
  size_t release(Object_registry *registry)
  bool fetch_schema_components(*schema, Const_ptr_vec<T> *coll)
  bool fetch_global_components(Const_ptr_vec<T> *coll)
  bool store(T *object)
  update(T *new_object)
  bool drop(const T *object)
}

title: database object representation 
class Table_ref  {
    char *db
    char *table_name
    LEX_CSTRING target_tablespace_name
    table_map m_map
    table_map sj_inner_tables
    Table_ref *natural_join
    List<Natural_join_column> *join_columns
    TABLE *table

    ST_SCHEMA_TABLE *schema_table
    Query_block *schema_query_block

    Query_block *query_block


    MDL_request mdl_request
}

class Weak_object <<virtual>>  {

}

class Entity_object<<virtual>> extends Weak_object {
    Object_id id()
    void set_name(name)
}

class Entity_object_impl implements Entity_object {
    Object_id m_id
    String_type m_name
}

class Abstract_table<<virtual>> extends Entity_object 

class Table<<virtual>> extends Abstract_table

class Abstract_table_impl extends Entity_object_impl,Abstract_table {
    Column_collection m_columns
    Object_id m_schema_id
}

class Table_impl extends Abstract_table_impl,Table {
    String_type m_engine
    String_type m_comment
    enum_row_format m_row_format
    enum_partition_type m_partition_type

    Index_collection m_indexes
    Partition_collection m_partitions

    Object_id m_collation_id
    Object_id m_tablespace_id
}

Table_ref --> Table

1.4. 1.1 DD Cache and persistance

Storage_adapter handling of access to persistent storage.


class Storage_adapter {
    Object_registry m_core_registry
    mysql_mutex_t m_lock

    void core_get(K &key, T **object)
    void core_store(THD *thd, T *object)
    bool core_sync(THD *thd, K &key, T *object)

    bool get(THD *thd, K &key, isolation,bypass_registry, T *object)
    bool store(THD *thd, T *object)
    bool drop(THD *thd, T *object)


}