>>> 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?
> 
> I am afraid of additional 'if's for such internal thing.
> 
>> 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()).
> 
> I think that on insertion into analyze spaces we correctly start a
> transaction, and it is not for region usage only. As I remember,
> there were problems with stat spaces consistency if we update stat
> not in a transaction. Region is just a convenient benefit.

It is not an insertion: OP_AnalysisLoad executes SELECT from
stat spaces, and before those SELECTs we start transaction.

>> Mb we always can avoid calling txn_begin_ro_stmt() on system spaces?
> 
> It is okay, but for ephemeral spaces too, it is not?

Then, I don’t understand why we can’t avoid calling it for any memtx space?
As far as I see (before this patch) we don’t call txn_begin_ro_stmt() at all.

> On the parsing
> stage we know is the space system one or not. Besides, I think that if
> a user calls explicit "SELECT * FROM _space", we should begin ro,
> but not when we open an internal cursor (ephemeral, or to insert into
> stats or something). It looks like a good question for the server
> team chat - should be start read-only statements on explicit SELECT
> from a system space? Box API now does it (inside box_index_iterator()).
> 
>> 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
> 
> Then I do not understand why can not we do the same for ANALYZE:
> 
> SELECT samples from different engines with no ro stmts;
> START TX;
> INSERT tuples into stat1/4;
> COMMIT;

Look how things happen:

1. SELECT samples from different engines INSERT INTO ephemeral space
(It doesn’t matter with or without ro stmt since there is no active transaction);
2. SELECT from ephemeral space INSERT INTO _stat spaces;
3. START TX;
4. SELECT FROM _stat space INSERT INTO tarantool data-dictionary
(i.e. fill index->def->opts->stat with current statistics);

Now, tarantool fails on last (4th) step due to obvious reason: SELECT
results in explicit 

> 
>>> 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;