[tarantool-patches] Re: [PATCH v1 1/1] sql: prevent executing crossengine sql

["n.pettik" <korablev@tarantool.org>]

>>  	struct iterator *it = index_create_iterator(*pCur->space->index,
>>  						    ITER_ALL, nil_key,
>>  						    0 /* part_count */);
>> @@ -1134,12 +1135,26 @@ cursor_seek(BtCursor *pCur, int *pRes)
>>  		return SQL_TARANTOOL_ITERATOR_FAIL;
>>  	}
>>  +	struct space *space = pCur->space;
>> +	struct txn *txn = NULL;> +
>> +	assert(space->def->id != 0 || space_is_memtx(space));
>> +	if (space->def->id != 0 &&> +	    space->def->id != BOX_SQL_STAT4_ID &&
>> +	    space->def->id != BOX_SQL_STAT1_ID) {
> 
> 2. I strongly do not like these 3 checks.
> 
> * space->id == 0 can be skipped, since begin_ro_stmt needs
> only space engine.
> 
> * id != stat4/1 is a huge crutch that should be removed. The
> later will be dilapidated by any stat or tx manager change.
> I guess it is because of ANALYZE that scans spaces and then
> updates _stat1/4. But I can not understand, why it can not
> firstly read the samples from user spaces, commit ro, and
> secondly insert the samples into stats in another transaction.
> Besides, I believe, that user spaces and analyze ones should
> not be in a single transaction ever as former are actually
> system ones and should not be mixed with user spaces.
> Nikita, I appeal to you for onlooking. Is it possible to
> split ANALYZE in two transactions? How could Kirill do it?
> Besides there is another option, but IMHO it could seem even
> more flawed.

Why do you think so? Actually, I can’t come up with better solution:
when we start executing OP_AnalysisLoad we always start transaction
(member that issue with gathering statistics on region to make it consistent?
See sql_analysis_load()).
Mb we always can avoid calling txn_begin_ro_stmt() on system spaces?

Quite similar to this issue might be CREATE TABLE AS SELECT
when it will be implemented. However, in this case we still can avoid
mixing user and system spaces within one transaction:

INSERT TO _SPACE/_INDEX etc
SELECT FROM source INSERT TO ephemeral
START TRANSACTION
SELECT FROM ephemeral INSERT TO destination
COMMIT TRANSACTION

> We could introduce a new opcode or add an option
> for OP_Seek like 'is_atomic' that will trigger ro stmt start.
> For spaces like analyze and ephemeral ones we could set this
> option in True thereby "allowing" cross-engine transactions
> for internal usage when we do not need read-view and other tx
> manager facilities.
> 
> 
>> +		if (txn_begin_ro_stmt(space, &txn) != 0)
>> +			return SQL_TARANTOOL_ERROR;
>> +	}
>>  	struct iterator *it = index_create_iterator(pCur->index, pCur->iter_type,
>>  						    key, part_count);
>>  	if (it == NULL) {
>> +		if (txn != NULL)
>> +			txn_rollback_stmt();
>>  		pCur->eState = CURSOR_INVALID;
>>  		return SQL_TARANTOOL_ITERATOR_FAIL;
>>  	}
>> +	if (txn != NULL)
>> +		txn_commit_ro_stmt(txn);
>>  	pCur->iter = it;
>>  	pCur->eState = CURSOR_VALID;
>>  
>