ColumnStore Information Schema Tables

1. COLUMNSTORE_TABLES "COLUMNSTORE_TABLES"

2. COLUMNSTORE_COLUMNS "COLUMNSTORE_COLUMNS"

3. COLUMNSTORE_EXTENTS "COLUMNSTORE_EXTENTS"

4. COLUMNSTORE_FILES "COLUMNSTORE_FILES"

5. Stored Procedures "Stored Procedures"

6. total_usage() "total_usage()"

7. table_usage() "table_usage()"

8. compression_ratio() "compression_ratio()"

MariaDB ColumnStore has four information schema tables that expose information about the table and column storage. The 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_SCHEMA.COLUMNSTORE_TABLES. It contains information about the tables inside ColumnStore.

COLUMNSTORE_COLUMNS

The INFORMATION_SCHEMA.COLUMNSTORE_COLUMNS table contains information about every single column inside ColumnStore.

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:

Column	Description
Column	Description
OBJECT_ID	The object ID for the extent
OBJECT_TYPE	Whether this is a "Column" or "Dictionary" extent
LOGICAL_BLOCK_START	ColumnStore's internal start LBID for this extent
LOGICAL_BLOCK_END	ColumnStore's internal end LBID for this extent
MIN_VALUE	This minimum value stored in this extent
MAX_VALUE	The maximum value stored in this extent
WIDTH	The data width for the extent
DBROOT	The DBRoot number for the extent
PARTITION_ID	The parition ID for the extent
SEGMENT_ID	The segment ID for the extent
BLOCK_OFFSET	The block offset for the data file, each data file can contain multiple extents for a column
MAX_BLOCKS	The maximum number of blocks for the extent
HIGH_WATER_MARK	The last block committed to the extent (starting at 0)
STATE	The state of the extent (see below)
STATUS	The availability status for the column which is either "Available", "Unavailable" or "Out of service"
DATA_SIZE	The uncompressed data size for the extent calculated as (HWM + 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 and create an extent file of 2561024WIDTH 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

5. HWM is set to zero for the lower segments when there are multiple segments in an extent file; these can be observed when BLOCK_OFFSET > 0

6. When HWM is 0, the DATA_SIZE will show 0 instead of 8192 to avoid confusion when there is multiple segments in an extent file

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.

Column	Description
Column	Description
OBJECT_ID	The object ID for the extent
SEGMENT_ID	The segment ID for the extent
PARTITION_ID	The partition ID for the extent
FILENAME	The full path and filename for the extent file, multiple extents for the same column can point to this file with different BLOCK_OFFSETs
FILE_SIZE	The disk file size for the extent
COMPRESSED_DATA_SIZE	The amount of the compressed file used, NULL if this is an uncompressed file

Stored Procedures

The total_usage() procedure gives a total disk usage summary for all the columns in ColumnStore except 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 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();

_{This page is licensed: CC BY-SA / Gnu FDL}