Undo
The GUC parameter of undo is valid only for the Ustore.
undo_space_limit_size
Parameter description: Specifies the threshold for forcibly recycling undo space. When the undo space usage reaches 80% of the threshold, forcible recycling starts. You are advised to set undo_space_limit_size to a value greater than or equal to that of undo_limit_size_per_transaction.
Parameter type: integer.
Unit: page (8 KB)
Value range: 102400 to 2147483647. The default unit is page.
Default value: 33554432 (that is, 256 GB)
Setting method: This is a SIGHUP parameter. Set it based on instructions provided in Table 1. For example, the value 1000000 indicates 1,000,000 pages, and the value 1000000kB indicates 1,000,000 KB. If the value contains a unit, the value can be KB, MB, or GB, but cannot be TB.
Setting suggestion: Retain the default value. If the disk space is too small to meet the default value requirements, set this parameter to a smaller value. If long transactions or large transactions exist in your service, set undo_space_limit_size to a larger value based on the disk usage. For details about the value setting, see the info column of the gs_stat_undo system function in "SQL Reference > Functions and Operators > System Administration Functions > Undo System Functions > gs_stat_undo parameters" in Developer Guide.
Risks and impacts of improper settings: If this parameter is set to a small value when undo records cannot be reclaimed in a timely manner, undo records may be forcibly reclaimed. As a result, there is an extremely low probability that the error message "snapshot too old! the undo record has been forcibly discard" is displayed. If this parameter is set to a large value when the disk space is insufficient and undo records cannot be reclaimed in a timely manner, a large number of undo records may be retained in the database system. As a result, the undo records occupy the physical disk space with other data, and services may be affected when the disk space is insufficient.
undo_limit_size_per_transaction
Parameter description: Specifies the undo space threshold of a single transaction. If the threshold is reached, the transaction is rolled back due to an error. You are advised to set undo_limit_size_per_transaction to a value less than or equal to that of undo_space_limit_size. If the value of undo_limit_size_per_transaction is greater than that of undo_space_limit_size, the displayed value is the same as the configured value when you run the show undo_limit_size_per_transaction command to query the parameter value. The only difference is that the smaller value between undo_space_limit_size and undo_limit_size_per_transaction is used as the actual undo space threshold of a single transaction.
Parameter type: integer.
Unit: page (8 KB)
Value range: 256 to 2147483647. The default unit is page. If this parameter is set to a value greater than 134217728 (that is, 1 TB), the value 134217728 takes effect.
Default value: 4194304 (32 GB)
Setting method: This is a SIGHUP parameter. Set it based on instructions provided in Table 1. For example, if the value is 10000, undo_limit_size_per_transaction is 10,000 pages. If the value is 10000kB, undo_limit_size_per_transaction is 10,000 KB. If the value contains a unit, the value can be KB, MB, or GB, but cannot be TB.
Setting suggestion: Retain the default value. If the disk or memory space is too small to meet the requirements of the default value, set this parameter to a smaller value. If long transactions or large transactions exist in your service, and the number of Undo records generated in a single transaction is greater than the value of this parameter, set undo_limit_size_per_transaction to a larger value based on the disk usage. For details about the value setting, see the info column of the gs_stat_undo system function in "SQL Reference > Functions and Operators > System Administration Functions > Undo System Functions > gs_stat_undo parameters" in Developer Guide. If undo_limit_size_per_transaction is set to a value greater than 1 TB, the system performance and stability may be affected. Therefore, if the parameter is set to a value greater than 134217728 (that is, 1 TB), the configured value is displayed when you run the show undo_limit_size_per_transaction command to query the parameter value, but the value 134217728 takes effect.
Risks and impacts of improper settings: If the parameter is set to small value, the number of undo records generated in a single transaction may exceed the threshold. As a result, the error message "undo size of the transaction exceeds the threshold" is displayed and the transaction is rolled back.
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