비교적 높은 격리 수준(5)으로서, 더티 읽기, 반복 불가능한 읽기가 발생하지 않지만, 유령 읽기는 발생할 수 있다.
다음과 같은 규칙이 적용된다.
이 격리 수준은 2단계 잠금 프로토콜을 따른다.
다음은 동시에 수행되는 트랜잭션의 격리 수준이 REPEATABLE READ CLASS with REPEATABLE READ INSTANCES인 경우 한 트랜잭션에서 객체 읽기를 수행하는 동안 다른 트랜잭션이 새로운 레코드를 추가할 수 있으므로 유령 읽기가 발생할 수 있음을 보여주는 예제이다.
session 1 |
session 2 |
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;autocommit off AUTOCOMMIT IS OFF
SET TRANSACTION ISOLATION LEVEL 5;
Isolation level set to: REPEATABLE READ SCHEMA, REPEATABLE READ INSTANCES. |
;autocommit off AUTOCOMMIT IS OFF
SET TRANSACTION ISOLATION LEVEL 5;
Isolation level set to: REPEATABLE READ SCHEMA, REPEATABLE READ INSTANCES. |
--creating a table
CREATE TABLE isol5_tbl(host_year integer, nation_code char(3)); CREATE UNIQUE INDEX on isol5_tbl(nation_code, host_year); INSERT INTO isol5_tbl VALUES (2008, 'AUS'); INSERT INTO isol5_tbl VALUES (2004, 'AUS');
COMMIT; |
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--selecting records from the table SELECT * FROM isol5_tbl WHERE nation_code='AUS'; host_year nation_code =================================== 2004 'AUS' 2008 'AUS' |
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INSERT INTO isol5_tbl VALUES (2004, 'KOR'); INSERT INTO isol5_tbl VALUES (2000, 'AUS');
/* able to insert new rows only when locks are not conflicted */ |
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SELECT * FROM isol5_tbl WHERE nation_code='AUS';
/* phantom read may occur when tran 1 committed */ |
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COMMIT; |
host_year nation_code =================================== 2000 'AUS' 2004 'AUS' 2008 'AUS' |
DELETE FROM isol5_tbl WHERE nation_code = 'AUS' and host_year=2008;
/* unable to delete rows until tran 2 committed */ |
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COMMIT; |
SELECT * FROM isol5_tbl WHERE nation_code = 'AUS';
/* unable to select rows until tran 1 committed */ |
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COMMIT; |
host_year nation_code =================================== 2000 'AUS' 2004 'AUS' |
ALTER TABLE isol5_tbl ADD COLUMN gold INT;
/* unable to alter the table schema until tran 2 committed */ |
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/* repeatable read is ensured while tran_1 is altering table schema */
SELECT * FROM isol5_tbl WHERE nation_code = 'AUS'; host_year nation_code =================================== 2000 'AUS' 2004 'AUS' |
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COMMIT; |
SELECT * FROM isol5_tbl WHERE nation_code = 'AUS'; /* unable to access the table until tran_1 committed */ |
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COMMIT; |
host_year nation_code gold =================================== 2000 'AUS' NULL 2004 'AUS' NULL |