DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD (Transact-SQL)

Artikkeli
03/03/2023

Displays the number of incremental changes in the base tables that are held for materialized views in Azure Synapse Analytics. The overhead ratio is calculated as TOTAL_ROWS / MAX (1, BASE_VIEW_ROWS).

Transact-SQL syntax conventions

Syntax

DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD ( "[ schema_name . ] materialized_view_name" )
[;]

Note

This syntax is not supported by serverless SQL pool in Azure Synapse Analytics.

Arguments

schema_name

The name of the schema to which the view belongs.

materialized_view_name

The name of the materialized view.

Remarks

To keep materialized views refreshed with data changes in base tables, data warehouse engine adds tracking rows to each affected view to reflect the changes. Selecting from a materialized view includes scanning the view's clustered columnstore index and applying any incremental changes.  The tracking rows (TOTAL_ROWS - BASE_VIEW_ROWS) don't get eliminated until users REBUILD the materialized view.

The overhead_ratio is calculated as TOTAL_ROWS / MAX(1, BASE_VIEW_ROWS). If it's high, SELECT performance will degrade. Users can rebuild the materialized view to reduce its overhead ratio.

Permissions

Requires VIEW DATABASE STATE permission.

Examples

A. This example returns the overhead ratio of a materialized view.

DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD ("dbo.MyIndexedView");

Sample output:

OBJECT_ID	BASE_VIEW_ROWS	TOTAL_ROWS	OVERHEAD_RATIO
1234	1	3	3.0

B. This example shows how the materialized view overhead increases as data changes in base tables

Create a table:

CREATE TABLE t1 (c1 INT NOT NULL, c2 INT NOT NULL, c3 INT NOT NULL);

Insert five rows into t1:

INSERT INTO t1 VALUES (1, 1, 1);
INSERT INTO t1 VALUES (2, 2, 2);
INSERT INTO t1 VALUES (3, 3, 3);
INSERT INTO t1 VALUES (4, 4, 4);
INSERT INTO t1 VALUES (5, 5, 5);

Create materialized views MV1:

CREATE MATERIALIZED VIEW MV1
WITH (DISTRIBUTION = HASH(c1))
AS
SELECT c1, COUNT(*) total_number
FROM dbo.t1 WHERE c1 < 3
GROUP BY c1;

Selecting from the materialized view returns two rows.

c1	total_number
1	1
2	1

Check the materialized view overhead before any data changes in the base table.

DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD ("dbo.mv1")

Sample output:

OBJECT_ID	BASE_VIEW_ROWS	TOTAL_ROWS	OVERHEAD_RATIO
587149137	2	2	1.00000000000000000

Update the base table. This query updates the same column in the same row 100 times to the same value. The materialized view content doesn't change.

DECLARE @p INT;
SELECT @p = 1;
WHILE (@p < 101)
BEGIN
    UPDATE t1 SET c1 = 1 WHERE c1 = 1;
    SELECT @p = @p + 1;
END;

Selecting from the materialized view returns the same result as before.

c1	total_number
1	1
2	1

Below is the output from DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD ("dbo.mv1"). 100 rows are added to the materialized view (total_row - base_view_rows) and its overhead_ratio is increased.

OBJECT_ID	BASE_VIEW_ROWS	TOTAL_ROWS	OVERHEAD_RATIO
587149137	2	102	51.00000000000000000

After rebuilding the materialized view, all tracking rows for incremental data changes are eliminated and the view overhead ratio is reduced.

ALTER MATERIALIZED VIEW dbo.MV1 REBUILD
GO
DBCC PDW_SHOWMATERIALIZEDVIEWOVERHEAD ("dbo.mv1")

Output

| OBJECT_ID | BASE_VIEW_ROWS | TOTAL_ROWS | OVERHEAD_RATIO |
| --- | --- | --- | --- |
| 587149137 | 2 | 2 | 1.00000000000000000 |

Jaa