MySQL 8.0 to improve the readability and execution efficiency of SQL statements. • Support changing column types online to improve code development flexibility. • Introduce a new statistics type to improve TiDB rolling upgrade. • Avoid executing statements like alter table ... modify column or alter table �→ ... change column during the TiDB rolling upgrade. • Since v5.1, setting the replica of system tables To enable it, see the user docu- ment. • Enhance the capability of changing column types online. Support changing the column type online using the ALTER TABLE statement, including but not limited to:

generated column or an expression index. The default value is OFF. tidb_opt_enable_correlation_adjustment �→ Newly added Controls whether the optimizer estimates the number of rows based on column order following enhancements are made to Lock View: – In addition to the SQL digest column in the Lock View-related tables, add a column to these tables that shows the corresponding normalized SQL text. You do not operation of querying the statements that have been historically executed by a transaction. – Add a column in the DATA_LOCK_WAITS and DEADLOCKS system tables to show the table name, row ID, index value, and

dynamic modification of logger.count and logger.size – Improve the data validation capability of column-based files (checksums, experi- mental feature) • Tools – TiCDC * Reduce the default value of planner might cache invalid plans for join in some cases #28087 – Fix wrong IndexLookUpJoin when hash column type is enum #27893 – Fix a batch client bug that recycling idle connection might block sending requests execution summaries �→ length for MPP queries #1791 – Fix the issue of error log cannot found column in Schema column for MPP queries #28149 – Fix the issue that TiDB might panic when TiFlash is shuting down

corresponding version. 2.2.1.1 System variables Variable name Change type Description tidb_enable_column_tracking �→ Newly added Controls whether to allow TiDB to collect PREDICATE �→ COLUMNS. The default default value is 5. TiDB stats-load �→ -queue- �→ size Newly added Controls the maximum number of column requests that the TiDB syn- chronously loading statistics feature can cache. The default value is1000 from INSTANCE or GLOBAL to GLOBAL #30756 • Forbid converting the integer type column to the time type column when the column contains zero #25728 • Fix the issue that no error is reported for the Inf or

tasks, and the task records persist after the restart. When tidb_analyze_version = 2, the Job_info column adds the analyze option infor- mation. • Damaged SST files in TiKV might cause the TiKV process task type) #33332 • Support using rand() as the default value of a column #10377 • Support using uuid() as the default value of a column #33870 • Support modifying the character set of columns from latin1 caused by out-of-bounds data in the auto_increment column #29737 • Fix the issue of local backend import failure when the auto_random or auto_increment column is null #34208 85 2.3 TiDB Features This document

threshold via tidb_server_memory_limit. • The high-performance and globally monotonic AUTO_INCREMENT column attribute be- comes GA, which is compatible with MySQL. • FLASHBACK CLUSTER TO TIMESTAMP is now optimizes the rewriting of COUNT(*) and automatically selects the not-null columns with the shortest column definition to count the number of rows, which can effectively 39 reduce the number of I/O operations 2.2.1.7 MySQL compatibility • Support a high-performance and globally monotonic AUTO_INCREMENT column attribute (GA) #38442 @tiancaiamao Since v6.4.0, TiDB has introduced the AUTO_INCREMENT MySQL compatibility

· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 6722 17.3.3 Column Family (CF) · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 6722 limitations because it does not support global indexes. For example, the unique key must use every column in the table’s partition expression. If the query condition does not use the partition key, the query address issues of Checksum V1 in verifying old values in Update or Delete events after ADD COLUMN or DROP COLUMN operations. For clusters created in v8.4.0 or later, or clusters upgraded to v8.4.0, TiDB

@AilinKid • Fix the issue that executing ALTER TABLE ... LAST PARTITION fails when the partition column type is DATETIME #48814 @crazycs520 • Fix the issue that using the _ wildcard in LIKE when the data tidb_server_memory_limit #48741 @XuHuaiyu • Fix the issue that the query result is incorrect when an ENUM type column is used as the join key #48991 @winoros • Fix the issue that queries containing CTEs unexpectedly @mjonss • Fix the issue that the START_TIME column type in INFORMATION_SCHEMA. �→ CLUSTER_INFO is not valid #45221 @dveeden • Fix the issue that invalid EXTRA column type in INFORMATION_SCHEMA.COLUMNS �→

ALTER. This method is limited in some scenarios. For example, the application allows ADD COLUMN but not DROP COLUMN, but they are both filtered by ALTER events in the earlier DM versions. To address such the granularity of the supported DDL events, such as support filtering MODIFY COLUMN (modify the column data type), DROP COLUMN, and other fine-grained DDL events that lead to data loss, truncation of data parti- tions. v7.5.0 in- troduces a new value option columns, which uses the explic- itly specified column values to calculate the partition number. 53 Configuration file Configuration parame- ter Change

�→ _blank">Generated columns (GA) Values in a generated column are calculated by a SQL expression in the �→ column definition in real time. This feature pushes some application �→ logic TiFlash first scans the column data related to the filter conditions that are pushed down to the TableScan operator, filters the rows that meet the condition, and then scans the other column data of these rows you perform a DDL operation that affects all rows on a large table (such as adding or deleting a column), the replication latency of TiCDC would significantly increase. Starting from v7.1.0, TiCDC optimizes