[HACKERS] pg_dump transaction's read-only mode

[Pavan Deolasee]

I'm looking at the following code in pg_dump.c

/*
     * Start transaction-snapshot mode transaction to dump consistent data.
     */
    ExecuteSqlStatement(fout, "BEGIN");
    if (fout->remoteVersion >= 90100)
    {
        if (serializable_deferrable)
            ExecuteSqlStatement(fout,
                                "SET TRANSACTION ISOLATION LEVEL "
                                "SERIALIZABLE, READ ONLY, DEFERRABLE");
        else
            ExecuteSqlStatement(fout,
                                "SET TRANSACTION ISOLATION LEVEL "
                                "REPEATABLE READ");
    }
    else
        ExecuteSqlStatement(fout,
                            "SET TRANSACTION ISOLATION LEVEL SERIALIZABLE");

Is there a reason why we do not the RR transaction as READ ONLY above
? I understand that unlike in the case of SERIALIZABLE transaction, it
may not have any performance impact. But isn't it a good practice
anyways to guard against any unintended database modification while
taking a dump or a safe guard against any future optimizations for
read-only transactions ? More so because RR seems to the default for
pg_dump

Thanks,
Pavan


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