Lock을 쓰는 것과 트랜잭션 고립수준을 높이는 것의 차이?

• MySQL(InnoDB)
• REPEATABLE READ

This is the default isolation level for InnoDB. Consistent reads within the same transaction read the snapshot established by the first read. This means that if you issue several plain (nonlocking) SELECT statements within the same transaction, these SELECT statements are consistent also with respect to each other. See Section 15.7.2.3, “Consistent Nonlocking Reads”.

For locking reads (SELECT with FOR UPDATE or FOR SHARE), UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index with a unique search condition, or a range-type search condition.

• For a unique index with a unique search condition, InnoDB locks only the index record found, not the gap before it.

• For other search conditions, InnoDB locks the index range scanned, using gap locks or next-key locks to block insertions by other sessions into the gaps covered by the range. For information about gap locks and next-key locks, see Section 15.7.1, “InnoDB Locking”.

• SERIALIZABLE

This level is like REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to SELECT ... FOR SHARE if autocommit is disabled. If autocommit is enabled, the SELECT is its own transaction. It therefore is known to be read only and can be serialized if performed as a consistent (nonlocking) read and need not block for other transactions. (To force a plain SELECT to block if other transactions have modified the selected rows, disable autocommit.)

• REPEATABLE READ 에서는 Unique Index 를 사용하는 SELECT 쿼리 혹은 특정 Unique 조건 검색인 경우에 대해 Lock을 건다. 또한 Range 검색이되는 경우엔 Gap Lock을 건다.
• SERIALIZABLE 에서는 REPEATABLE READ 와 다르게 모든 SELECT 쿼리에서 Lock 을 건다. (autocommit 옵션이 빠진 경우)

• PostgreSQL
• READ COMMITTED (PostgreSQL 은 고립수준이 Read Comitted 가 기본이다)
 
MySQL :: MySQL 8.0 Reference Manual :: 15.7.2.1 Transaction Isolation Levels
Transaction isolation is one of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time.

https://dev.mysql.com/doc/refman/8.0/en/innodb-transaction-isolation-levels.html
13.2. Transaction Isolation
13.2. Transaction Isolation # 13.2.1. Read Committed Isolation Level 13.2.2. Repeatable Read Isolation Level 13.2.3. Serializable Isolation Level The SQL standard …

https://www.postgresql.org/docs/current/transaction-iso.html#XACT-READ-COMMITTED

요약