![]() ![]() However, such a target row might have already been updated (or deleted or locked) by another concurrent transaction by the time it is found. UPDATE, DELETE, SELECT FOR UPDATE, and SELECT FOR SHARE commands behave the same as SELECT in terms of searching for target rows: they will only find target rows that were committed as of the command start time. Also note that two successive SELECT commands can see different data, even though they are within a single transaction, if other transactions commit changes after the first SELECT starts and before the second SELECT starts. However, SELECT does see the effects of previous updates executed within its own transaction, even though they are not yet committed. In effect, a SELECT query sees a snapshot of the database as of the instant the query begins to run. ![]() When a transaction uses this isolation level, a SELECT query (without a FOR UPDATE/SHARE clause) sees only data committed before the query began it never sees either uncommitted data or changes committed during query execution by concurrent transactions. Read Committed is the default isolation level in PostgreSQL. To set the transaction isolation level of a transaction, use the command SET TRANSACTION. The behavior of the available isolation levels is detailed in the following subsections. This is acceptable under the SQL standard because the standard specifies which anomalies must not occur at certain isolation levels higher guarantees are acceptable. The table also shows that PostgreSQL's Repeatable Read implementation does not allow phantom reads. This is because it is the only sensible way to map the standard isolation levels to PostgreSQL's multiversion concurrency control architecture. In PostgreSQL, you can request any of the four standard transaction isolation levels, but internally only three distinct isolation levels are implemented, i.e., PostgreSQL's Read Uncommitted mode behaves like Read Committed. ![]()
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