2.9.2 Transaction isolation levels supported by HADB

HADB supports the following transaction isolation levels:

READ COMMITTED
REPEATABLE READ

Organization of this subsection

(1) READ COMMITTED
(2) REPEATABLE READ

(1) READ COMMITTED

Data that has already been committed when an SQL statement is executed (when a cursor is opened for retrieval) becomes the processing target of the SQL statement. Therefore, if the same SELECT statement is executed multiple times within the same transaction, data updating by another transaction might cause the execution result of the SELECT statement to vary.

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(2) REPEATABLE READ

If the same SQL statement with the same search condition is executed repeatedly within a transaction for which the REPEATABLE READ isolation level is set, the data that becomes the processing target of that SQL statement is always the same regardless of any processing by other transactions.

The data to be processed by the SQL statement changes according to the conditions shown in the following tables.

Table 2‒9: Relationship between chunks that store data and the data to be processed by an SQL statement
Table 2‒10: Relationship between a transaction that updated the data within a chunk and the data to be processed by an SQL statement

Table 2‒9: Relationship between chunks that store data and the data to be processed by an SQL statement
No.	Chunks that store data	Data to be processed by an SQL statement
1	Chunk that was created during base table definition Chunk that was created during execution of the `TRUNCATE` `TABLE` statement Chunk that was created during execution of the `adbimport` command with the `-d` option specified Chunk that was created during execution of the `adbimport` command, which uses the background-import facility before executing a local transaction Chunk that was created during execution of the `adbmergechunk` command (merge-target chunk) Chunk whose status was changed by the `adbchgchunkstatus` command Archived chunk that was created when the `adbimport` command, which uses the background-import facility, was executed in a local transaction	For details, see Table 2‒10: Relationship between a transaction that updated the data within a chunk and the data to be processed by an SQL statement.
2	Non-archived chunk that was created when the `adbimport` command, which uses the background-import facility, was executed in a local transaction	None^#

#

If the following two commands are executed in a local transaction, the data to be processed by SQL statements changes as shown in Table 2‒9: Relationship between chunks that store data and the data to be processed by an SQL statement and Table 2‒10: Relationship between a transaction that updated the data within a chunk and the data to be processed by an SQL statement.

The adbimport command, which uses the background-import facility
The adbmergechunk command, which merges multiple chunks, including the chunk created during execution of the adbimport command

The following figure shows an example.

Figure 2‒34: Changes in the data to be processed by the local transaction in connection with command execution

Table 2‒10: Relationship between a transaction that updated the data within a chunk and the data to be processed by an SQL statement
No.	Transaction that updated the data within a chunk		Data that becomes the processing target of the SQL statement
1	Local transaction		All data
2	Other transaction	Updating with an SQL statement: `INSERT` statement `UPDATE` statement `DELETE` statement	Data that had already been committed when the local transaction started
3		Adding data with the `adbimport` command	Data when the `adbimport` command terminates normally
4		Updating with the `PURGE` `CHUNK` statement	Data when the `PURGE` `CHUNK` statement terminates normally
5		Updating with the `TRUNCATE` `TABLE` statement	Data when the `TRUNCATE` `TABLE` statement terminates normally
6		Changing the chunk status with the `adbchgchunkstatus` command	Data when the `adbchgchunkstatus` command terminates normally

Important: If a retrieval with a chunk ID specified (for example, the #GETDATA subcommand of the adbsql command) is executed, the transaction isolation level might not become REPEATABLE READ. For details, see (4) Chunks that are retrieved during execution of the adbmergechunk command in 11.4.9 Merging chunks (to reduce the number of chunks).

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