TiFlash Late Materialization

This document describes how to use the TiFlash late materialization feature to accelerate queries in OLAP scenarios.

By default, when receiving a query request, TiFlash reads all the data from the columns required by the query, and then filters and aggregates the data based on the query conditions. Late materialization is an optimization method that supports pushing down part of the filter conditions to the TableScan operator. That is, TiFlash first scans the column data related to the filter conditions that are pushed down, filters the rows that meet the condition, and then scans the other column data of these rows for further calculation, thereby reducing IO scans and computations of data processing.

If you want to improve the performance of certain queries in OLAP scenarios, you can enable the TiFlash late materialization feature at the session level or global level. By modifying the value of the tidb_opt_enable_late_materialization system variable, you can choose to enable or disable the TiFlash late materialization feature.

When the TiFlash late materialization feature is enabled, the TiDB optimizer will determine which filter conditions will be pushed down based on statistics and filter conditions. The optimizer will prioritize pushing down the filter conditions with high filtration rates. For detailed algorithms, see the RFC document.

For example:

EXPLAIN SELECT a, b, c FROM t1 WHERE a < 1;

+-------------------------+----------+--------------+---------------+-------------------------------------------------------+
| id                      | estRows  | task         | access object | operator info                                         |
+-------------------------+----------+--------------+---------------+-------------------------------------------------------+
| TableReader_12          | 12288.00 | root         |               | MppVersion: 1, data:ExchangeSender_11                 |
| └─ExchangeSender_11     | 12288.00 | mpp[tiflash] |               | ExchangeType: PassThrough                             |
|   └─TableFullScan_9     | 12288.00 | mpp[tiflash] | table:t1      | pushed down filter:lt(test.t1.a, 1), keep order:false |
+-------------------------+----------+--------------+---------------+-------------------------------------------------------+

In this example, the filter condition a < 1 is pushed down to the TableScan operator. TiFlash first reads all data from column a, and then filters the rows that meet the a < 1 condition. Next, TiFlash reads columns b and c from these filtered rows.

Enable or disable TiFlash late materialization

By default, the tidb_opt_enable_late_materialization system variable is OFF at both the session and global levels, which means that the TiFlash late materialization feature is not enabled. You can use the following statement to view the corresponding variable information:

SHOW VARIABLES LIKE 'tidb_opt_enable_late_materialization';

+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| tidb_opt_enable_late_materialization | OFF   |
+--------------------------------------+-------+

SHOW GLOBAL VARIABLES LIKE 'tidb_opt_enable_late_materialization';

+--------------------------------------+-------+
| Variable_name                        | Value |
+--------------------------------------+-------+
| tidb_opt_enable_late_materialization | OFF   |
+--------------------------------------+-------+

You can modify the tidb_opt_enable_late_materialization variable at the session level or at the global level.

• To enable TiFlash late materialization in the current session, use the following statement:

  SET SESSION tidb_opt_enable_late_materialization=ON;
  

• To enable TiFlash late materialization at the global level, use the following statement:

  SET GLOBAL tidb_opt_enable_late_materialization=ON;
  

  After this setting, the tidb_opt_enable_late_materialization variable will be enabled by default for both session and global levels in new sessions.

To disable TiFlash late materialization, use the following statements:

SET SESSION tidb_opt_enable_late_materialization=OFF;

SET GLOBAL tidb_opt_enable_late_materialization=OFF;

Implementation mechanism

When filter conditions are pushed down to the TableScan operator, the execution process of the TableScan operator mainly includes the following steps:

1. Reads the three columns <handle, del_mark, version>, performs multi-version concurrency control (MVCC) filtering, and then generates the MVCC Bitmap.
2. Reads the columns related to the filter conditions, filters the rows that meet the conditions, and then generates the Filter Bitmap.
3. Performs an AND operation between the MVCC Bitmap and Filter Bitmap to generate the Final Bitmap.
4. Reads the corresponding rows of the remaining columns according to the Final Bitmap.
5. Merges the data read in steps 2 and 4, and then returns the results.