The User Module manages and controls the operation of end-user queries. It maintains the state of each query, issues requests to one or more Performance Modules to perform work on behalf of a query, and performs end resolution of a query by aggregating the various result sets from all participating Performance Modules into one that is ultimately returned to the end user.
The User Module contains several processes:
- The MariaDB Server process: mysqld
- This mysqld is the modified MariaDB Server binaries that comes specifically with MariaDB ColumnStore.
- It does the same normal tasks as MariaDB Server - connection validation, parsing of SQL statements, general SQL plan generation, final result set distribution activities.
- Additionally it converts the MariaDB Server query plan into the MariaDB ColumnStore query plan format. The MariaDB ColumnStore query plan format is still essentially a parse tree, but execution hints from the optimizer are added to assist the User Module in converting the parse tree to a Job List.
- Execution Manager: ExeMgr
- ExeMgr listens on a TCP/IP port for query parse trees from mysqld. ExeMgr is responsible for converting the query parse tree into a job list, which is a construct in MariaDB ColumnStore that represents the sequence of instructions necessary to answer the query. ExeMgr walks the query parse tree and iteratively generates job steps, optimizing and re-optimizing the job list as it goes. The major categories of job steps are application of a column filter, processing a table join, and projection of returned columns. Each operation in a query plan is executed in parallel by the job list itself and has the capability of running entirely on the User Module, entirely on the Performance Module or in some combination. Each node uses the Extent Map to determine which Performance Modules to send work orders to (see ColumnStore Storage Architecture for details on the Extent).
- DML, DDL and import distribution managers (DMLProc, DDLProc and cpimport).
- DMLProc and DDLProc distribute DML and DDL to the appropriate Performance Module. Cpimport, when run on the User Module, distributes source files to the Performance Modules.
The User Module performs these core functions for MariaDB ColumnStore:
- Transforms MariaDB plan into a ColumnStore Job List.
- Performs InfiniDB OID (object ID) lookups from the MariaDB ColumnStore system catalog.
- Inspects the Extent Map (described in ColumnStore Storage Architecture) to reduce I/O, which is accomplished via the elimination of unnecessary extents.
- Issues instructions (sometimes referred to as ‘primitive operations’) to the Performance Modules.
- Executes hash joins as needed (depending on size of smaller tables in the join). Helps manage distributed hash joins by sending any hash maps needing processing to Performance Modules.
- Executes cross-table-scope functions and expressions that happen after a hash join.
- Receives data from the Performance Modules and re-transmits back to them if needed.
- Executes follow-up steps for all aggregation and distinct processing.
- Returns data back to the MariaDB interface. The primary job of the User Module is to handle concurrency scaling. It never directly touches database files and does not require visibility to them. It uses a machine’s RAM in a transitory manner to assemble partial query results into a complete answer that is ultimately returned to a user.