MariaDB ColumnStore has four Information Schema tables that expose information about the table and column storage. These tables were added in version 1.0.5 of ColumnStore and were heavily modified for 1.0.6.

COLUMNSTORE_TABLES

The first table is the INFORMATION_SCHMEA.COLUMNSTORE_TABLES. This contains information about the tables inside ColumnStore. The table layout is as follows:

ColumnDescription
TABLE_SCHEMAThe database schema for the table
TABLE_NAMEThe table name
OBJECT_IDThe ColumnStore object ID for the table
CREATION_DATEThe date the table was created
COLUMN_COUNTThe number of columns in the table
AUTOINCREMENTThe start autoincrement value for the table set during CREATE TABLE

Note: Tables created with ColumnStore 1.0.4 or lower will have the year field of the creation data set incorrectly by 1900 years.

COLUMNSTORE_COLUMNS

The INFORMATION_SCHEMA.COLUMNSTORE_COLUMNS table contains information about every single column inside ColumnStore. The table layout is as follows:

ColumnDescription
TABLE_SCHEMAThe database schema for the table
TABLE_NAMEThe table name for the column
COLUMN_NAMEThe column name
OBJECT_IDThe object ID for the column
DICTIONARY_OBJECT_IDThe dictionary object ID for the column (NULL if there is no dictionary object
LIST_OBJECT_IDPlaceholder for future information
TREE_OBJECT_IDPlaceholder for future information
DATA_TYPEThe data type for the column
COLUMN_LENGTHThe data length for the column
COLUMN_POSITIONThe position of the column in the table, starting at 0
COLUMN_DEFAULTThe default value for the column
IS_NULLABLEWhether or not the column can be set to NULL
NUMERIC_PRECISIONThe numeric precision for the column
NUMERIC_SCALEThe numeric scale for the column
IS_AUTOINCREMENTSet to 1 if the column is an autoincrement column
COMPRESSION_TYPEThe type of compression (either "None" or "Snappy")

COLUMNSTORE_EXTENTS

This table displays the extent map in a user consumable form. An extent is a collection of details about a section of data related to a columnstore column. A majority of columns in ColumnStore will have multiple extents and the columns table above can be joined to this one to filter results by table or column. The table layout is as follows:

ColumnDescription
OBJECT_IDThe object ID for the extent
OBJECT_TYPEWhether this is a "Column" or "Dictionary" extent
LOGICAL_BLOCK_STARTColumnStore's internal start LBA for this extent
LOGICAL_BLOCK_ENDColumnStore's internal end LBA for this extent
MIN_VALUEThis minimum value stored in this extent
MAX_VALUEThe maximum value stored in this extent
WIDTHThe data width for the extent
DBROOTThe DBRoot number for the extent
PARTITION_IDThe parition ID for the extent
SEGMENT_IDThe segment ID for the extent
BLOCK_OFFSETThe block offset for the data file, each data file can contain multiple extents for a column
MAX_BLOCKSThe maximum number of blocks for the extent
HIGH_WATER_MARKThe last block committed to the extent (starting at 0)
STATEThe state of the extent (see below)
STATUSThe availability status for the column which is either "Available", "Unavailable" or "Out of service"
DATA_SIZEThe uncompressed data size for the extent calculated as (HWM - BLOCK_OFFSET + 1) * BLOCK_SIZE

Notes:

  1. The state is "Valid" for a normal state, "Invalid" if a cpimport has completed but the table has not yet been accessed (min/max values will be invalid) or "Updating" if there is a DML statement writing to the column
  2. In ColumnStore the block size is 8192 bytes
  3. By default ColumnStore will write create an extent file of 256*1024*WIDTH bytes for the first partition, if this is too small then for uncompressed data it will create a file of the maximum size for the extent (MAX_BLOCKS * BLOCK_SIZE). Snappy always compression adds a header block.
  4. Object IDs of less than 3000 are for internal tables and will not appear in any of the information schema tables

COLUMNSTORE_FILES

The columnstore_files table provides information about each file associated with extensions. Each extension can reuse a file at different block offsets so this is not a 1:1 relationship to the columnstore_extents table.

ColumnDescription
OBJECT_IDThe object ID for the extent
SEGMENT_IDThe segment ID for the extent
PARTITION_IDThe partition ID for the extent
FILENAMEThe full path and filename for the extent file, multiple extents for the same column can point to this file with different BLOCK_OFFSETs
FILE_SIZEThe disk file size for the extent
COMPRESSED_DATA_SIZEThe amount of the compressed file used, NULL if this is an uncompressed file

Stored Procedures

A few stored procedures were added in 1.0.6 to provide summaries based on the information schema tables. These can be accessed from the COLUMNSTORE_INFO schema.

total_usage()

The total_usage() procedure gives a total disk usage summary for all the columns in ColumnStore with the exception of the columns used for internal maintenance. It is executed using the following query:

> call columnstore_info.total_usage();

table_usage()

The table_usage() procedure gives a the total data disk usage, dictionary disk usage and grand total disk usage per-table. It can be called in several ways, the first gives a total for each table:

> call columnstore_info.table_usage(NULL, NULL);

Or for a specific table, my_table in my_schema in this example:

> call columnstore_info.table_usage('my_schema', 'my_table');

You can also request all tables for a specified schema:

> call columnstore_info.table_usage('my_schema', NULL);

Note: The quotes around the table name are required, an error will occur without them.

compression_ratio()

The compression_ratio() procedure calculates the average compression ratio across all the compressed extents in ColumnStore. It is called using:

> call columnstore_info.compression_ratio();

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