[HACKERS] Statement-level rollback

Hello,

As I stated here and at the PGConf.ASIA developer meeting last year, I'd like to propose statement-level rollback feature.  To repeat myself, this is requested for users to migrate from other DBMSs to PostgreSQL.  They expect that a failure of one SQL statement should not abort the entire transaction and their apps (client programs and stored procedures) can continue the transaction with a different SQL statement.


SPECIFICATION
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START TRANSACTION ROLLBACK SCOPE { TRANSACTION | STATEMENT };

This syntax controls the behavior of the transaction when an SQL statement fails.  TRANSACTION (default) is the traditional behavior (i.e. rolls back the entire transaction or subtransaction).  STATEMENT rolls back the failed SQL statement.

Just like the isolation level and access mode, default_transaction_rollback_scope GUC variable is also available.


DESIGN
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Nothing much to talk about... it merely creates a savepoint before each statement execution and destroys it after the statement finishes.  This is done in postgres.c for top-level SQL statements.

The stored function hasn't been handled yet; I'll submit the revised patch soon.


CONSIDERATIONS AND REQUESTS
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The code for stored functions is not written yet, but I'd like your feedback for the specification and design based on the current patch.  I'll add this patch to CommitFest 2017-3.

The patch creates and destroys a savepoint for each message of the extended query protocol (Parse, Bind, Execute and Describe).  I'm afraid this will add significant overhead, but I don't find a better way, because those messages could be send arbitrarily for different statements, e.g. Parse stmt1, Parse stmt2, Bind stmt1, Execute stmt1, Bind stmt2, Execute stmt2.


Regards
Takayuki Tsunakawa


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