[Tarantool-patches] [PATCH v3 2/5] box: introdice _vsession_settings sysview

[imeevma@tarantool.org]

Hi! Thank you for review! My answers and new patch below.

On 11/2/19 7:17 PM, Vladislav Shpilevoy wrote:
> Hi! Thanks for the fixes!
>
> See 7 comments below.
>
>> diff --git a/src/box/session.cc b/src/box/session.cc
>> index d69b657..6d301bf 100644
>> --- a/src/box/session.cc
>> +++ b/src/box/session.cc
>> @@ -361,3 +364,135 @@ generic_session_sync(struct session *session)
>>  	(void) session;
>>  	return 0;
>>  }
>> +
>> +int
>> +session_options_get(struct index *index, const char *key, uint32_t part_count,
>> +		    struct tuple **result)
>> +{
>> +	assert(part_count == 1);
>> +	(void)part_count;
>> +	struct space *space = space_cache_find(index->def->space_id);
>> +	uint32_t len;
>> +	const char *name = mp_decode_str(&key, &len);
>> +	name = tt_cstr(name, len);
>> +	/*
>> +	 * Currently, the only session local options are SQL
>> +	 * options.
>> +	 */
>> +	uint32_t id_max = sql_option_id_max();
>> +	for (uint32_t id = 0; id < id_max; ++id) {
>> +		if (sql_option_compare(name, id) == 0)
>> +			return sql_option_tuple(space->format, id, result);
>> +	}
>> +	*result = NULL;
>> +	return 0;
>> +}
>> +
>> +/**
>> + * An iterator that iterates over current session options.
>> + */
>> +struct session_options_iterator {
>> +	/** Base iterator. Must be the first member. */
>> +	struct iterator base;
>> +	/** Format of the tuples this iterator returns. */
>> +	struct tuple_format *format;
>> +	/** Counter, count number of options already shown. */
>> +	uint32_t counter;
>> +	/** Decoded key. */
>> +	char *key;
>> +	/** Type of iterator. */
>> +	enum iterator_type iterator_type;
>> +};
>> +
>> +static int
>> +session_options_iterator_next(struct iterator *itr, struct tuple **ret)
>> +{
>> +	struct session_options_iterator *it =
>> +		(struct session_options_iterator *)itr;
>> +	uint32_t id_max = sql_option_id_max();
>> +	if (it->key == NULL)
>> +		return sql_option_tuple(it->format, it->counter++, ret);
>> +	for (; it->counter < id_max; ++it->counter) {
>> +		int compare = sql_option_compare(it->key, it->counter);
>> +		if (compare == 0 && (it->iterator_type == ITER_EQ ||
>> +				     it->iterator_type == ITER_GE ||
>> +				     it->iterator_type == ITER_ALL))
>> +			break;
>> +		if (compare > 0 && (it->iterator_type == ITER_GT ||
>> +				    it->iterator_type == ITER_GE ||
>> +				    it->iterator_type == ITER_ALL))
>> +			break;
>> +	}
>> +	if (it->counter < id_max)
>> +		return sql_option_tuple(it->format, it->counter++, ret);
>> +	*ret = NULL;
>> +	return 0;
>> +}
>> +
>> +static int
>> +session_options_iterator_prev(struct iterator *itr, struct tuple **ret)
>> +{
>> +	struct session_options_iterator *it =
>> +		(struct session_options_iterator *)itr;
>> +	uint32_t id_max = sql_option_id_max();
>> +	if (it->key == NULL)
>> +		return sql_option_tuple(it->format, id_max - 1 - it->counter++,
>> +					ret);
>> +	for (; it->counter < id_max; ++it->counter) {
>> +		uint32_t id = id_max - it->counter - 1;
>
> 1. These 'id_max - 1 - counter' look really complex. Could you
> simplify it? For example, rename it to 'current_id', and in
> iterator_prev() decrement it instead of increment.
>
Fixed.

>> +		int compare = sql_option_compare(it->key, id);
>> +		if (compare == 0 && (it->iterator_type == ITER_REQ ||
>> +				     it->iterator_type == ITER_LE))
>> +			break;
>> +		if (compare < 0 && (it->iterator_type == ITER_LT ||
>> +				    it->iterator_type == ITER_LE))
>> +			break;
>> +	}
>> +	if (it->counter < id_max)
>> +		return sql_option_tuple(it->format, id_max - 1 - it->counter++,
>> +					ret);
>> +	*ret = NULL;
>> +	return 0;
>> +}
>> +
>> +static void
>> +session_options_iterator_free(struct iterator *itr)
>> +{
>> +	struct session_options_iterator *it =
>> +		(struct session_options_iterator *)itr;
>> +	free(it->key);
>> +	free(it);
>> +}
>> +
>> +struct iterator *
>> +session_options_create_iterator(struct index *index, enum iterator_type type,
>> +				const char *key, uint32_t part_count)
>> +{
>> +	char *decoded_key = NULL;
>> +	if (part_count > 0) {
>> +		assert(part_count == 1);
>> +		assert(mp_typeof(*key) == MP_STR);
>> +		uint32_t len;
>> +		const char *name = mp_decode_str(&key, &len);
>> +		name = tt_cstr(name, len);
>> +		decoded_key = (char *)malloc(len + 1);
>
> 2. I guess, you don't need tt_cstr() here, if you anyway
> copy the key onto the heap memory.
>
Fixed.

> 3. You don't check malloc() result.
>
Fixed.

>> +		memcpy(decoded_key, name, len + 1);
>> +	}
>> +	struct space *space = space_cache_find(index->def->space_id);
>> +	struct session_options_iterator *it =
>> +		(session_options_iterator *)malloc(sizeof(*it));
>> +	if (it == NULL) {
>> +		diag_set(OutOfMemory, sizeof(*it), "malloc", "it");
>> +		return NULL;
>> +	}
>> +	iterator_create(&it->base, index);
>> +	it->base.next = iterator_type_is_reverse(type) ?
>> +			session_options_iterator_prev :
>> +			session_options_iterator_next;
>> +	it->base.free = session_options_iterator_free;
>> +	it->key = decoded_key;
>> +	it->iterator_type = type;
>> +	it->format = space->format;
>> +	it->counter = 0;
>> +	return (struct iterator *)it;
>> +}
>> diff --git a/src/box/sql/build.c b/src/box/sql/build.c
>> index 51cd7ce..dceb092 100644
>> --- a/src/box/sql/build.c
>> +++ b/src/box/sql/build.c
>> @@ -3242,3 +3243,151 @@ sql_fieldno_by_name(struct Parse *parse_context, struct Expr *field_name,
>>  	*fieldno = i;
>>  	return 0;
>>  }
>> +
>> +/**
>> + * Identifiers of all SQL options that can be viewed. The
>> + * identifier of the option is equal to its place in the sorted
>> + * list of options, which starts at 0.
>> + *
>> + * It is IMPORTANT that SQL options are sorted by name. If this is
>> + * not the case, the result returned by the _vsession_settings
>> + * space iterator will not be sorted properly.
>> + */
>> +enum {
>> +	SQL_OPTION_COMPOUND_SELECT_LIMIT = 0,
>> +	SQL_OPTION_DEFAULT_ENGINE,
>> +	SQL_OPTION_DEFER_FOREIGN_KEYS,
>> +	SQL_OPTION_FULL_COLUMN_NAMES,
>> +#ifndef NDEBUG
>> +	SQL_OPTION_PARSER_TRACE,
>> +#endif
>> +	SQL_OPTION_RECURSIVE_TRIGGERS,
>> +	SQL_OPTION_REVERSE_UNORDERED_SELECTS,
>> +#ifndef NDEBUG
>> +	SQL_OPTION_SELECT_TRACE,
>> +	SQL_OPTION_TRACE,
>> +	SQL_OPTION_VDBE_ADDOPTRACE,
>> +	SQL_OPTION_VDBE_DEBUG,
>> +	SQL_OPTION_VDBE_EQP,
>> +	SQL_OPTION_VDBE_LISTING,
>> +	SQL_OPTION_VDBE_TRACE,
>> +	SQL_OPTION_WHERE_TRACE,
>> +#endif
>> +	SQL_OPTION_max,
>> +};
>> +
>> +/**
>> + * A local structure that allows to establish a connection between
>> + * the name of the parameter, its field type and mask, if it have
>> + * one.
>> + */
>> +struct sql_option_metadata
>> +{
>> +	const char *name;
>> +	uint32_t field_type;
>> +	uint32_t mask;
>> +};
>> +
>> +/**
>> + * Variable that contains names of the SQL options, their field
>> + * types and mask if they have one or 0 if don't have.
>> + *
>> + * It is IMPORTANT that these options sorted by name.
>> + */
>> +static struct sql_option_metadata sql_options[] = {
>> +	/** SQL_OPTION_COMPOUND_SELECT_LIMIT */
>> +	{"sql_compound_select_limit", FIELD_TYPE_INTEGER, 0},
>> +	/** SQL_OPTION_DEFAULT_ENGINE */
>> +	{"sql_default_engine", FIELD_TYPE_STRING, 0},
>> +	/** SQL_OPTION_DEFER_FOREIGN_KEYS */
>> +	{"sql_defer_foreign_keys", FIELD_TYPE_BOOLEAN, SQL_DeferFKs},
>> +	/** SQL_OPTION_FULL_COLUMN_NAMES */
>> +	{"sql_full_column_names", FIELD_TYPE_BOOLEAN, SQL_FullColNames},
>> +#ifndef NDEBUG
>> +	/** SQL_OPTION_PARSER_TRACE */
>> +	{"sql_parser_trace", FIELD_TYPE_BOOLEAN, PARSER_TRACE_FLAG},
>> +#endif
>> +	/** SQL_OPTION_RECURSIVE_TRIGGERS */
>> +	{"sql_recursive_triggers", FIELD_TYPE_BOOLEAN, SQL_RecTriggers},
>> +	/** SQL_OPTION_REVERSE_UNORDERED_SELECTS */
>> +	{"sql_reverse_unordered_selects", FIELD_TYPE_BOOLEAN, SQL_ReverseOrder},
>> +#ifndef NDEBUG
>> +	/** SQL_OPTION_SELECT_TRACE */
>> +	{"sql_select_trace", FIELD_TYPE_BOOLEAN, SQL_SelectTrace},
>> +	/** SQL_OPTION_TRACE */
>> +	{"sql_trace", FIELD_TYPE_BOOLEAN, SQL_SqlTrace},
>> +	/** SQL_OPTION_VDBE_ADDOPTRACE */
>> +	{"sql_vdbe_addoptrace", FIELD_TYPE_BOOLEAN, SQL_VdbeAddopTrace},
>> +	/** SQL_OPTION_VDBE_DEBUG */
>> +	{"sql_vdbe_debug", FIELD_TYPE_BOOLEAN,
>> +	 SQL_SqlTrace | SQL_VdbeListing | SQL_VdbeTrace},
>> +	/** SQL_OPTION_VDBE_EQP */
>> +	{"sql_vdbe_eqp", FIELD_TYPE_BOOLEAN, SQL_VdbeEQP},
>> +	/** SQL_OPTION_VDBE_LISTING */
>> +	{"sql_vdbe_listing", FIELD_TYPE_BOOLEAN, SQL_VdbeListing},
>> +	/** SQL_OPTION_VDBE_TRACE */
>> +	{"sql_vdbe_trace", FIELD_TYPE_BOOLEAN, SQL_VdbeTrace},
>> +	/** SQL_OPTION_WHERE_TRACE */
>> +	{"sql_where_trace", FIELD_TYPE_BOOLEAN, SQL_WhereTrace},
>> +#endif
>> +};
>> +
>> +uint32_t
>> +sql_option_id_max()
>> +{
>> +	return SQL_OPTION_max;
>> +}
>> +
>> +int
>> +sql_option_compare(const char *key, uint32_t id)
>> +{
>> +	assert(id < SQL_OPTION_max);
>> +	return strcmp(sql_options[id].name, key);
>> +}
>> +
>> +int
>> +sql_option_tuple(struct tuple_format *format, int option_id,
>> +		 struct tuple **result)
>> +{
>> +	if (option_id < 0 || option_id >= SQL_OPTION_max) {
>> +		*result = NULL;
>> +		return 0;
>> +	}
>> +	int limit = 0;
>> +	const char *engine = NULL;
>> +	struct session *session = current_session();
>> +	uint32_t flags = session->sql_flags;
>> +	uint32_t mask = sql_options[option_id].mask;
>> +	/* Tuple format contains two fields - name and value. */
>> +	uint32_t column_count = format->min_field_count;
>> +	assert(column_count == 2);
>> +	size_t size = mp_sizeof_array(column_count) +
>> +		      mp_sizeof_str(strlen(sql_options[option_id].name));
>> +	if (sql_options[option_id].field_type == FIELD_TYPE_BOOLEAN) {
>> +		size += mp_sizeof_bool(true);
>> +	} else if (option_id == SQL_OPTION_DEFAULT_ENGINE) {
>> +		engine = sql_storage_engine_strs[session->sql_default_engine];
>> +		size += mp_sizeof_str(strlen(engine));
>> +	} else {
>> +		assert(option_id == SQL_OPTION_COMPOUND_SELECT_LIMIT);
>> +		limit = sql_get()->aLimit[SQL_LIMIT_COMPOUND_SELECT];
>> +		size += mp_sizeof_uint(limit);
>
> 4. I propose you to remove compound select setting from there. Since
> it is not session local.
>
Done.

> Another question is where to show its value then? Certainly not
> here.
>
>> +	}
>> +
>> +	char *pos_ret = static_alloc(size);
>> +	assert(pos_ret != NULL);
>> +	char *pos = mp_encode_array(pos_ret, column_count);
>> +	pos = mp_encode_str(pos, sql_options[option_id].name,
>> +			    strlen(sql_options[option_id].name));
>> +	if (sql_options[option_id].field_type == FIELD_TYPE_BOOLEAN)
>> +		pos = mp_encode_bool(pos, (flags & mask) == mask);
>> +	else if (option_id == SQL_OPTION_DEFAULT_ENGINE)
>> +		pos = mp_encode_str(pos, engine, strlen(engine));
>> +	else
>> +		pos = mp_encode_uint(pos, limit);
>> +	struct tuple *tuple = tuple_new(format, pos_ret, pos_ret + size);
>> +	if (tuple == NULL)
>> +		return -1;
>> +	*result = tuple;
>> +	return 0;
>> +}
>> diff --git a/test/box/misc.result b/test/box/misc.result
>> index b293051..1edd59e 100644
>> --- a/test/box/misc.result
>> +++ b/test/box/misc.result
>> @@ -1330,3 +1330,174 @@ test_run:grep_log('default', 'test log')
>>  ---
>>  - test log
>>  ...
>> +--
>> +-- gh-4511: make sure that _vsession_settings sysview works as
>> +-- intended.
>
> 5. I just realized that probably Kirill would want it to be in a
> separate file. But if he doesn't, then probably access_sysview.test.lua
> fits better than misc.
>
Fixed, moved to access_sysview.test.lua.

>> +--
>> +v = box.space._vsession_settings
>> +---
>> +...
>> +option_count = v:count()
>> +---
>> +...
>> +v:format()
>> +---
>> +- [{'name': 'name', 'type': 'string'}, {'name': 'value', 'type': 'any'}]
>> +...
>> +(#v:select()) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'ALL'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'REQ'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'EQ'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'GE'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'GT'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'LE'})) == option_count
>> +---
>> +- true
>> +...
>> +(#v:select({}, {iterator = 'LT'})) == option_count
>
> 6. Why do you need braces around #v:select?
>
I thought that this looked easier to understand, but guess it is
not so.

>> +---
>> +- true
>> +...
>> +s = box.schema.space.create('settings', {format = v:format()})
>> +---
>> +...
>> +_ = s:create_index('primary')
>> +---
>> +...
>> +for _,value in v:pairs() do s:insert(value) end
>> +---
>> +...
>> +(#v:select({'abcde'}, {iterator = 'ALL'})) == (#s:select({'abcde'}, {iterator = 'ALL'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'REQ'})) == (#s:select({'abcde'}, {iterator = 'REQ'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'EQ'})) == (#s:select({'abcde'}, {iterator = 'EQ'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'GE'})) == (#s:select({'abcde'}, {iterator = 'GE'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'GT'})) == (#s:select({'abcde'}, {iterator = 'GT'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'LE'})) == (#s:select({'abcde'}, {iterator = 'LE'}))
>> +---
>> +- true
>> +...
>> +(#v:select({'abcde'}, {iterator = 'LT'})) == (#s:select({'abcde'}, {iterator = 'LT'}))
>> +---
>> +- true
>
> 7. Sorry, but counts are rather useless here, they don't check order nor content.
> Please, check them properly. Also there is still no a single test about partial
> keys. You check for empty, full, not found, but not for partial matches like 'sql_v'
> + GE/LE/GT/LT. Which already appeared to be buggy two times.
>
> I propose you to implement a function, which takes a key, and checks
> it with different iterator types on both 's' and 'v', and compares their result
> sets. Then you call it with different keys and without tests code duplication.
>
> Additionally, you could find useful gcov build to check code coverage locally.
> Use make distclean && cmake . -DENABLE_GCOV=1 && make -j. Then run a needed
> test. Then call
>
>     gcov <path_to_session.cc> -o <path_to_session.gcno>
>
> Then a file session.gcov is generated where you can check whether you have
> covered your code with tests properly. File session.gcno should appear somewhere
> after you run a test.
>

Done, gcov after access_sysview.test.lua:
        -:  367:
        -:  368:int
        5:  369:session_options_get(struct index *index, const char *key, uint32_t part_count,
        -:  370:		    struct tuple **result)
        -:  371:{
        5:  372:	assert(part_count == 1);
        -:  373:	(void)part_count;
        5:  374:	struct space *space = space_cache_find(index->def->space_id);
        -:  375:	uint32_t len;
        5:  376:	const char *name = mp_decode_str(&key, &len);
        5:  377:	name = tt_cstr(name, len);
        -:  378:	/*
        -:  379:	 * Currently, the only session local options are SQL
        -:  380:	 * options.
        -:  381:	 */
        5:  382:	uint32_t id_max = sql_session_opt_id_max();
       29:  383:	for (uint32_t id = 0; id < id_max; ++id) {
       28:  384:		if (sql_session_opt_compare(name, id) == 0)
        4:  385:			return sql_session_opt_tuple(space->format, id, result);
        -:  386:	}
        1:  387:	*result = NULL;
        1:  388:	return 0;
        -:  389:}
        -:  390:
        -:  391:/**
        -:  392: * An iterator that iterates over current session options.
        -:  393: */
        -:  394:struct session_options_iterator {
        -:  395:	/** Base iterator. Must be the first member. */
        -:  396:	struct iterator base;
        -:  397:	/** Format of the tuples this iterator returns. */
        -:  398:	struct tuple_format *format;
        -:  399:	/** ID of the option in global list of the options. */
        -:  400:	int current_id;
        -:  401:	/** Decoded key. */
        -:  402:	char *key;
        -:  403:	/** Type of iterator. */
        -:  404:	enum iterator_type iterator_type;
        -:  405:};
        -:  406:
        -:  407:static int
      264:  408:session_options_iterator_next(struct iterator *itr, struct tuple **ret)
        -:  409:{
        -:  410:	struct session_options_iterator *it =
      264:  411:		(struct session_options_iterator *)itr;
      264:  412:	if (it->key == NULL)
      105:  413:		return sql_session_opt_tuple(it->format, it->current_id++, ret);
      159:  414:	int id_max = sql_session_opt_id_max();
      351:  415:	for (; it->current_id < id_max; ++it->current_id) {
      238:  416:		int compare = sql_session_opt_compare(it->key, it->current_id);
      241:  417:		if (compare == 0 && (it->iterator_type == ITER_EQ ||
        5:  418:				     it->iterator_type == ITER_GE ||
        2:  419:				     it->iterator_type == ITER_ALL))
        -:  420:			break;
      385:  421:		if (compare > 0 && (it->iterator_type == ITER_GT ||
      256:  422:				    it->iterator_type == ITER_GE ||
      105:  423:				    it->iterator_type == ITER_ALL))
        -:  424:			break;
        -:  425:	}
      159:  426:	return sql_session_opt_tuple(it->format, it->current_id++, ret);
        -:  427:}
        -:  428:
        -:  429:static int
       77:  430:session_options_iterator_prev(struct iterator *itr, struct tuple **ret)
        -:  431:{
        -:  432:	struct session_options_iterator *it =
       77:  433:		(struct session_options_iterator *)itr;
       77:  434:	if (it->key == NULL)
       45:  435:		return sql_session_opt_tuple(it->format, it->current_id--, ret);
      328:  436:	for (; it->current_id >= 0; --it->current_id) {
      168:  437:		int compare = sql_session_opt_compare(it->key, it->current_id);
      170:  438:		if (compare == 0 && (it->iterator_type == ITER_REQ ||
        2:  439:				     it->iterator_type == ITER_LE))
        -:  440:			break;
      184:  441:		if (compare < 0 && (it->iterator_type == ITER_LT ||
       18:  442:				    it->iterator_type == ITER_LE))
        -:  443:			break;
        -:  444:	}
       32:  445:	return sql_session_opt_tuple(it->format, it->current_id--, ret);
        -:  446:}
        -:  447:
        -:  448:static void
       37:  449:session_options_iterator_free(struct iterator *itr)
        -:  450:{
        -:  451:	struct session_options_iterator *it =
       37:  452:		(struct session_options_iterator *)itr;
       37:  453:	free(it->key);
       37:  454:	free(it);
       37:  455:}
        -:  456:
        -:  457:struct iterator *
       39:  458:session_options_create_iterator(struct index *index, enum iterator_type type,
        -:  459:				const char *key, uint32_t part_count)
        -:  460:{
       39:  461:	char *decoded_key = NULL;
       39:  462:	if (part_count > 0) {
       29:  463:		assert(part_count == 1);
       58:  464:		assert(mp_typeof(*key) == MP_STR);
        -:  465:		uint32_t len;
       29:  466:		const char *name = mp_decode_str(&key, &len);
       29:  467:		decoded_key = (char *)malloc(len + 1);
       29:  468:		if (decoded_key == NULL) {
    #####:  469:			diag_set(OutOfMemory, len + 1, "malloc", "decoded_key");
    #####:  470:			return NULL;
        -:  471:		}
       29:  472:		memcpy(decoded_key, name, len);
       29:  473:		decoded_key[len] = '\0';
        -:  474:	}
       39:  475:	struct space *space = space_cache_find(index->def->space_id);
        -:  476:	struct session_options_iterator *it =
       39:  477:		(session_options_iterator *)malloc(sizeof(*it));
       39:  478:	if (it == NULL) {
    #####:  479:		diag_set(OutOfMemory, sizeof(*it), "malloc", "it");
    #####:  480:		return NULL;
        -:  481:	}
       39:  482:	int current_id = iterator_type_is_reverse(type) ?
       39:  483:			 sql_session_opt_id_max() - 1 : 0;
       39:  484:	iterator_create(&it->base, index);
       39:  485:	it->base.next = iterator_type_is_reverse(type) ?
        -:  486:			session_options_iterator_prev :
       39:  487:			session_options_iterator_next;
       39:  488:	it->base.free = session_options_iterator_free;
       39:  489:	it->key = decoded_key;
       39:  490:	it->iterator_type = type;
       39:  491:	it->format = space->format;
       39:  492:	it->current_id = current_id;
       39:  493:	return (struct iterator *)it;
       12:  494:}


New patch:

From 3d55476778a0b8be41703ccdc4d25d558b861dfd Mon Sep 17 00:00:00 2001
From: Mergen Imeev <imeevma@gmail.com>
Date: Thu, 24 Oct 2019 15:18:09 +0300
Subject: [PATCH] box: introdice _vsession_settings sysview

This patch creates the _vsession_settings sysview. This sysview
contains names and values of the session-local options.

This sysview creates tuples on the fly using its own get() and
create_iterator() methods. This allows us to get a tuple without
having to save it anywhere.

Currently, only SQL options can be extracted from this sysview,
since the only session-local options are SQL options.

Part of #4511

diff --git a/src/box/bootstrap.snap b/src/box/bootstrap.snap
index 59f6cc1..6b4b96a 100644
Binary files a/src/box/bootstrap.snap and b/src/box/bootstrap.snap differ
diff --git a/src/box/lua/space.cc b/src/box/lua/space.cc
index f6e96f0..9b5df28 100644
--- a/src/box/lua/space.cc
+++ b/src/box/lua/space.cc
@@ -653,6 +653,8 @@ box_lua_space_init(struct lua_State *L)
 	lua_setfield(L, -2, "SPACE_SEQUENCE_ID");
 	lua_pushnumber(L, BOX_FUNC_INDEX_ID);
 	lua_setfield(L, -2, "FUNC_INDEX_ID");
+	lua_pushnumber(L, BOX_VSESSION_SETTINGS_ID);
+	lua_setfield(L, -2, "VSESSION_SETTINGS_ID");
 	lua_pushnumber(L, BOX_SYSTEM_ID_MIN);
 	lua_setfield(L, -2, "SYSTEM_ID_MIN");
 	lua_pushnumber(L, BOX_SYSTEM_ID_MAX);
diff --git a/src/box/lua/upgrade.lua b/src/box/lua/upgrade.lua
index e71b7fb..94e4118 100644
--- a/src/box/lua/upgrade.lua
+++ b/src/box/lua/upgrade.lua
@@ -943,6 +943,28 @@ local function upgrade_to_2_3_0()
 end
 
 --------------------------------------------------------------------------------
+-- Tarantool 2.3.1
+--------------------------------------------------------------------------------
+
+local function create_vsession_settings_sysview()
+    local _space = box.space[box.schema.SPACE_ID]
+    local _index = box.space[box.schema.INDEX_ID]
+    local format = {}
+    format[1] = {name='name', type='string'}
+    format[2] = {name='value', type='any'}
+    log.info("create space _vsession_settings")
+    _space:insert{box.schema.VSESSION_SETTINGS_ID, ADMIN, '_vsession_settings',
+                  'sysview', 0, setmap({}), format}
+    log.info("create index _vsession_settings:primary")
+    _index:insert{box.schema.VSESSION_SETTINGS_ID, 0, 'primary', 'tree',
+                  {unique = true}, {{0, 'string'}}}
+end
+
+local function upgrade_to_2_3_1()
+    create_vsession_settings_sysview()
+end
+
+--------------------------------------------------------------------------------
 
 local function get_version()
     local version = box.space._schema:get{'version'}
@@ -977,6 +999,7 @@ local function upgrade(options)
         {version = mkversion(2, 1, 3), func = upgrade_to_2_1_3, auto = true},
         {version = mkversion(2, 2, 1), func = upgrade_to_2_2_1, auto = true},
         {version = mkversion(2, 3, 0), func = upgrade_to_2_3_0, auto = true},
+        {version = mkversion(2, 3, 1), func = upgrade_to_2_3_1, auto = true},
     }
 
     for _, handler in ipairs(handlers) do
diff --git a/src/box/schema_def.h b/src/box/schema_def.h
index ba870ff..b9824cb 100644
--- a/src/box/schema_def.h
+++ b/src/box/schema_def.h
@@ -114,6 +114,8 @@ enum {
 	BOX_CK_CONSTRAINT_ID = 364,
 	/** Space id of _func_index. */
 	BOX_FUNC_INDEX_ID = 372,
+	/** Space id of _vsession_settings. */
+	BOX_VSESSION_SETTINGS_ID = 381,
 	/** End of the reserved range of system spaces. */
 	BOX_SYSTEM_ID_MAX = 511,
 	BOX_ID_NIL = 2147483647
diff --git a/src/box/session.cc b/src/box/session.cc
index b171e23..d9e3b2d 100644
--- a/src/box/session.cc
+++ b/src/box/session.cc
@@ -36,6 +36,9 @@
 #include "user.h"
 #include "error.h"
 #include "tt_static.h"
+#include "index.h"
+#include "schema.h"
+#include "sql.h"
 
 const char *session_type_strs[] = {
 	"background",
@@ -361,3 +364,131 @@ generic_session_sync(struct session *session)
 	(void) session;
 	return 0;
 }
+
+int
+session_options_get(struct index *index, const char *key, uint32_t part_count,
+		    struct tuple **result)
+{
+	assert(part_count == 1);
+	(void)part_count;
+	struct space *space = space_cache_find(index->def->space_id);
+	uint32_t len;
+	const char *name = mp_decode_str(&key, &len);
+	name = tt_cstr(name, len);
+	/*
+	 * Currently, the only session local options are SQL
+	 * options.
+	 */
+	uint32_t id_max = sql_session_opt_id_max();
+	for (uint32_t id = 0; id < id_max; ++id) {
+		if (sql_session_opt_compare(name, id) == 0)
+			return sql_session_opt_tuple(space->format, id, result);
+	}
+	*result = NULL;
+	return 0;
+}
+
+/**
+ * An iterator that iterates over current session options.
+ */
+struct session_options_iterator {
+	/** Base iterator. Must be the first member. */
+	struct iterator base;
+	/** Format of the tuples this iterator returns. */
+	struct tuple_format *format;
+	/** ID of the option in global list of the options. */
+	int current_id;
+	/** Decoded key. */
+	char *key;
+	/** Type of iterator. */
+	enum iterator_type iterator_type;
+};
+
+static int
+session_options_iterator_next(struct iterator *itr, struct tuple **ret)
+{
+	struct session_options_iterator *it =
+		(struct session_options_iterator *)itr;
+	if (it->key == NULL)
+		return sql_session_opt_tuple(it->format, it->current_id++, ret);
+	int id_max = sql_session_opt_id_max();
+	for (; it->current_id < id_max; ++it->current_id) {
+		int compare = sql_session_opt_compare(it->key, it->current_id);
+		if (compare == 0 && (it->iterator_type == ITER_EQ ||
+				     it->iterator_type == ITER_GE ||
+				     it->iterator_type == ITER_ALL))
+			break;
+		if (compare > 0 && (it->iterator_type == ITER_GT ||
+				    it->iterator_type == ITER_GE ||
+				    it->iterator_type == ITER_ALL))
+			break;
+	}
+	return sql_session_opt_tuple(it->format, it->current_id++, ret);
+}
+
+static int
+session_options_iterator_prev(struct iterator *itr, struct tuple **ret)
+{
+	struct session_options_iterator *it =
+		(struct session_options_iterator *)itr;
+	if (it->key == NULL)
+		return sql_session_opt_tuple(it->format, it->current_id--, ret);
+	for (; it->current_id >= 0; --it->current_id) {
+		int compare = sql_session_opt_compare(it->key, it->current_id);
+		if (compare == 0 && (it->iterator_type == ITER_REQ ||
+				     it->iterator_type == ITER_LE))
+			break;
+		if (compare < 0 && (it->iterator_type == ITER_LT ||
+				    it->iterator_type == ITER_LE))
+			break;
+	}
+	return sql_session_opt_tuple(it->format, it->current_id--, ret);
+}
+
+static void
+session_options_iterator_free(struct iterator *itr)
+{
+	struct session_options_iterator *it =
+		(struct session_options_iterator *)itr;
+	free(it->key);
+	free(it);
+}
+
+struct iterator *
+session_options_create_iterator(struct index *index, enum iterator_type type,
+				const char *key, uint32_t part_count)
+{
+	char *decoded_key = NULL;
+	if (part_count > 0) {
+		assert(part_count == 1);
+		assert(mp_typeof(*key) == MP_STR);
+		uint32_t len;
+		const char *name = mp_decode_str(&key, &len);
+		decoded_key = (char *)malloc(len + 1);
+		if (decoded_key == NULL) {
+			diag_set(OutOfMemory, len + 1, "malloc", "decoded_key");
+			return NULL;
+		}
+		memcpy(decoded_key, name, len);
+		decoded_key[len] = '\0';
+	}
+	struct space *space = space_cache_find(index->def->space_id);
+	struct session_options_iterator *it =
+		(session_options_iterator *)malloc(sizeof(*it));
+	if (it == NULL) {
+		diag_set(OutOfMemory, sizeof(*it), "malloc", "it");
+		return NULL;
+	}
+	int current_id = iterator_type_is_reverse(type) ?
+			 sql_session_opt_id_max() - 1 : 0;
+	iterator_create(&it->base, index);
+	it->base.next = iterator_type_is_reverse(type) ?
+			session_options_iterator_prev :
+			session_options_iterator_next;
+	it->base.free = session_options_iterator_free;
+	it->key = decoded_key;
+	it->iterator_type = type;
+	it->format = space->format;
+	it->current_id = current_id;
+	return (struct iterator *)it;
+}
diff --git a/src/box/session.h b/src/box/session.h
index eff3d7a..f5e3cf2 100644
--- a/src/box/session.h
+++ b/src/box/session.h
@@ -36,12 +36,15 @@
 #include "fiber.h"
 #include "user.h"
 #include "authentication.h"
+#include "iterator_type.h"
 
 #if defined(__cplusplus)
 extern "C" {
 #endif /* defined(__cplusplus) */
 
 struct port;
+struct index;
+struct tuple;
 struct session_vtab;
 
 void
@@ -347,6 +350,16 @@ generic_session_fd(struct session *session);
 int64_t
 generic_session_sync(struct session *session);
 
+/** Method get() for _vsession_settings sysview. */
+int
+session_options_get(struct index *index, const char *key, uint32_t part_count,
+		    struct tuple **result);
+
+/** Method create_iterator() for _vsession_settings sysview. */
+struct iterator *
+session_options_create_iterator(struct index *index, enum iterator_type type,
+				const char *key, uint32_t part_count);
+
 #if defined(__cplusplus)
 } /* extern "C" */
 
diff --git a/src/box/sql.h b/src/box/sql.h
index 0fa52fc..604c3d2 100644
--- a/src/box/sql.h
+++ b/src/box/sql.h
@@ -70,6 +70,7 @@ struct Select;
 struct Table;
 struct sql_trigger;
 struct space_def;
+struct tuple_format;
 
 /**
  * Perform parsing of provided expression. This is done by
@@ -420,6 +421,41 @@ void
 vdbe_field_ref_prepare_tuple(struct vdbe_field_ref *field_ref,
 			     struct tuple *tuple);
 
+/**
+ * Return the next number after the ID of the last SQL session
+ * option.
+ *
+ * @retval Next number after the last SQL option.
+ */
+uint32_t
+sql_session_opt_id_max();
+
+/**
+ * Return an integer less than, equal to, or greater than zero if
+ * name of session option with given ID is found, respectively,
+ * to be less than, to match, or be greater than key.
+ *
+ * @param key Key to compare with option name.
+ * @param id ID of the option.
+ * @retval result of comparison.
+ */
+int
+sql_session_opt_compare(const char *key, uint32_t id);
+
+/**
+ * Create a tuple that contains the name and value of the SQL
+ * session option.
+ *
+ * @param format Format of the tuple.
+ * @param option_id ID of SQL option.
+ * @param[out] result New tuple or NULL.
+ * @retval 0 On success.
+ * @retval < 0 On error.
+ */
+int
+sql_session_opt_tuple(struct tuple_format *format, int option_id,
+		      struct tuple **result);
+
 #if defined(__cplusplus)
 } /* extern "C" { */
 #endif
diff --git a/src/box/sql/build.c b/src/box/sql/build.c
index 51cd7ce..ce87b88 100644
--- a/src/box/sql/build.c
+++ b/src/box/sql/build.c
@@ -48,6 +48,7 @@
 #include "vdbeInt.h"
 #include "tarantoolInt.h"
 #include "box/box.h"
+#include "box/tuple.h"
 #include "box/ck_constraint.h"
 #include "box/fk_constraint.h"
 #include "box/sequence.h"
@@ -3242,3 +3243,142 @@ sql_fieldno_by_name(struct Parse *parse_context, struct Expr *field_name,
 	*fieldno = i;
 	return 0;
 }
+
+/**
+ * Identifiers of all SQL session options that can be viewed. The
+ * identifier of the option is equal to its place in the sorted
+ * list of session options, which starts at 0.
+ *
+ * It is IMPORTANT that these options are sorted by name. If this
+ * is not the case, the result returned by the _vsession_settings
+ * space iterator will not be sorted properly.
+ */
+enum {
+	SQL_SESSION_OPTION_DEFAULT_ENGINE = 0,
+	SQL_SESSION_OPTION_DEFER_FOREIGN_KEYS,
+	SQL_SESSION_OPTION_FULL_COLUMN_NAMES,
+#ifndef NDEBUG
+	SQL_SESSION_OPTION_PARSER_TRACE,
+#endif
+	SQL_SESSION_OPTION_RECURSIVE_TRIGGERS,
+	SQL_SESSION_OPTION_REVERSE_UNORDERED_SELECTS,
+#ifndef NDEBUG
+	SQL_SESSION_OPTION_SELECT_TRACE,
+	SQL_SESSION_OPTION_TRACE,
+	SQL_SESSION_OPTION_VDBE_ADDOPTRACE,
+	SQL_SESSION_OPTION_VDBE_DEBUG,
+	SQL_SESSION_OPTION_VDBE_EQP,
+	SQL_SESSION_OPTION_VDBE_LISTING,
+	SQL_SESSION_OPTION_VDBE_TRACE,
+	SQL_SESSION_OPTION_WHERE_TRACE,
+#endif
+	SQL_SESSION_OPTION_max,
+};
+
+/**
+ * A local structure that allows to establish a connection between
+ * the name of the parameter, its field type and mask, if it have
+ * one.
+ */
+struct sql_option_metadata
+{
+	const char *name;
+	uint32_t field_type;
+	uint32_t mask;
+};
+
+/**
+ * Variable that contains names of the SQL session options, their
+ * field types and mask if they have one or 0 if don't have.
+ *
+ * It is IMPORTANT that these options sorted by name.
+ */
+static struct sql_option_metadata sql_session_opts[] = {
+	/** SQL_SESSION_OPTION_DEFAULT_ENGINE */
+	{"sql_default_engine", FIELD_TYPE_STRING, 0},
+	/** SQL_SESSION_OPTION_DEFER_FOREIGN_KEYS */
+	{"sql_defer_foreign_keys", FIELD_TYPE_BOOLEAN, SQL_DeferFKs},
+	/** SQL_SESSION_OPTION_FULL_COLUMN_NAMES */
+	{"sql_full_column_names", FIELD_TYPE_BOOLEAN, SQL_FullColNames},
+#ifndef NDEBUG
+	/** SQL_SESSION_OPTION_PARSER_TRACE */
+	{"sql_parser_trace", FIELD_TYPE_BOOLEAN, PARSER_TRACE_FLAG},
+#endif
+	/** SQL_SESSION_OPTION_RECURSIVE_TRIGGERS */
+	{"sql_recursive_triggers", FIELD_TYPE_BOOLEAN, SQL_RecTriggers},
+	/** SQL_SESSION_OPTION_REVERSE_UNORDERED_SELECTS */
+	{"sql_reverse_unordered_selects", FIELD_TYPE_BOOLEAN, SQL_ReverseOrder},
+#ifndef NDEBUG
+	/** SQL_SESSION_OPTION_SELECT_TRACE */
+	{"sql_select_trace", FIELD_TYPE_BOOLEAN, SQL_SelectTrace},
+	/** SQL_SESSION_OPTION_TRACE */
+	{"sql_trace", FIELD_TYPE_BOOLEAN, SQL_SqlTrace},
+	/** SQL_SESSION_OPTION_VDBE_ADDOPTRACE */
+	{"sql_vdbe_addoptrace", FIELD_TYPE_BOOLEAN, SQL_VdbeAddopTrace},
+	/** SQL_SESSION_OPTION_VDBE_DEBUG */
+	{"sql_vdbe_debug", FIELD_TYPE_BOOLEAN,
+	 SQL_SqlTrace | SQL_VdbeListing | SQL_VdbeTrace},
+	/** SQL_SESSION_OPTION_VDBE_EQP */
+	{"sql_vdbe_eqp", FIELD_TYPE_BOOLEAN, SQL_VdbeEQP},
+	/** SQL_SESSION_OPTION_VDBE_LISTING */
+	{"sql_vdbe_listing", FIELD_TYPE_BOOLEAN, SQL_VdbeListing},
+	/** SQL_SESSION_OPTION_VDBE_TRACE */
+	{"sql_vdbe_trace", FIELD_TYPE_BOOLEAN, SQL_VdbeTrace},
+	/** SQL_SESSION_OPTION_WHERE_TRACE */
+	{"sql_where_trace", FIELD_TYPE_BOOLEAN, SQL_WhereTrace},
+#endif
+};
+
+uint32_t
+sql_session_opt_id_max()
+{
+	return SQL_SESSION_OPTION_max;
+}
+
+int
+sql_session_opt_compare(const char *key, uint32_t id)
+{
+	assert(id < SQL_SESSION_OPTION_max);
+	return strcmp(sql_session_opts[id].name, key);
+}
+
+int
+sql_session_opt_tuple(struct tuple_format *format, int option_id,
+		 struct tuple **result)
+{
+	if (option_id < 0 || option_id >= SQL_SESSION_OPTION_max) {
+		*result = NULL;
+		return 0;
+	}
+	struct session *session = current_session();
+	uint32_t flags = session->sql_flags;
+	uint32_t mask = sql_session_opts[option_id].mask;
+	const char *engine = NULL;
+	/* Tuple format contains two fields - name and value. */
+	uint32_t column_count = format->min_field_count;
+	assert(column_count == 2);
+	size_t size = mp_sizeof_array(column_count) +
+		      mp_sizeof_str(strlen(sql_session_opts[option_id].name));
+	if (sql_session_opts[option_id].field_type == FIELD_TYPE_BOOLEAN) {
+		size += mp_sizeof_bool(true);
+	} else {
+		assert(option_id == SQL_SESSION_OPTION_DEFAULT_ENGINE);
+		engine = sql_storage_engine_strs[session->sql_default_engine];
+		size += mp_sizeof_str(strlen(engine));
+	}
+
+	char *pos_ret = static_alloc(size);
+	assert(pos_ret != NULL);
+	char *pos = mp_encode_array(pos_ret, column_count);
+	pos = mp_encode_str(pos, sql_session_opts[option_id].name,
+			    strlen(sql_session_opts[option_id].name));
+	if (sql_session_opts[option_id].field_type == FIELD_TYPE_BOOLEAN)
+		pos = mp_encode_bool(pos, (flags & mask) == mask);
+	else
+		pos = mp_encode_str(pos, engine, strlen(engine));
+	struct tuple *tuple = tuple_new(format, pos_ret, pos_ret + size);
+	if (tuple == NULL)
+		return -1;
+	*result = tuple;
+	return 0;
+}
diff --git a/src/box/sysview.c b/src/box/sysview.c
index 745cf09..83a8ccc 100644
--- a/src/box/sysview.c
+++ b/src/box/sysview.c
@@ -411,7 +411,7 @@ vsequence_filter(struct space *source, struct tuple *tuple)
 }
 
 static bool
-vcollation_filter(struct space *source, struct tuple *tuple)
+generic_filter(struct space *source, struct tuple *tuple)
 {
 	(void) source;
 	(void) tuple;
@@ -481,10 +481,17 @@ sysview_space_create_index(struct space *space, struct index_def *def)
 	case BOX_VCOLLATION_ID:
 		source_space_id = BOX_COLLATION_ID;
 		source_index_id = def->iid;
-		filter = vcollation_filter;
+		filter = generic_filter;
 		get = index_get;
 		create_iterator = index_create_iterator;
 		break;
+	case BOX_VSESSION_SETTINGS_ID:
+		source_space_id = BOX_VSESSION_SETTINGS_ID;
+		source_index_id = def->iid;
+		filter = generic_filter;
+		get = session_options_get;
+		create_iterator = session_options_create_iterator;
+		break;
 	default:
 		diag_set(ClientError, ER_MODIFY_INDEX,
 			 def->name, space_name(space),
@@ -569,9 +576,10 @@ sysview_engine_create_space(struct engine *engine, struct space_def *def,
 		return NULL;
 	}
 	struct tuple_format *format =
-		tuple_format_new(NULL, NULL, keys, key_count, def->fields,
-				 def->field_count, def->exact_field_count,
-				 def->dict, def->opts.is_temporary,
+		tuple_format_new(&tuple_format_runtime->vtab, NULL, keys,
+				 key_count, def->fields, def->field_count,
+				 def->exact_field_count, def->dict,
+				 def->opts.is_temporary,
 				 def->opts.is_ephemeral);
 	if (format == NULL) {
 		free(space);
diff --git a/test/app-tap/tarantoolctl.test.lua b/test/app-tap/tarantoolctl.test.lua
index f388208..a41fad3 100755
--- a/test/app-tap/tarantoolctl.test.lua
+++ b/test/app-tap/tarantoolctl.test.lua
@@ -410,8 +410,8 @@ do
             check_ctlcat_xlog(test_i, dir, "--from=3 --to=6 --format=json --show-system --replica 1", "\n", 3)
             check_ctlcat_xlog(test_i, dir, "--from=3 --to=6 --format=json --show-system --replica 1 --replica 2", "\n", 3)
             check_ctlcat_xlog(test_i, dir, "--from=3 --to=6 --format=json --show-system --replica 2", "\n", 0)
-            check_ctlcat_snap(test_i, dir, "--space=280", "---\n", 24)
-            check_ctlcat_snap(test_i, dir, "--space=288", "---\n", 52)
+            check_ctlcat_snap(test_i, dir, "--space=280", "---\n", 25)
+            check_ctlcat_snap(test_i, dir, "--space=288", "---\n", 53)
         end)
     end)
 
diff --git a/test/box-py/bootstrap.result b/test/box-py/bootstrap.result
index 123aa2f..54b43ec 100644
--- a/test/box-py/bootstrap.result
+++ b/test/box-py/bootstrap.result
@@ -4,7 +4,7 @@ box.internal.bootstrap()
 box.space._schema:select{}
 ---
 - - ['max_id', 511]
-  - ['version', 2, 3, 0]
+  - ['version', 2, 3, 1]
 ...
 box.space._cluster:select{}
 ---
@@ -96,6 +96,8 @@ box.space._space:select{}
         'type': 'boolean'}]]
   - [372, 1, '_func_index', 'memtx', 0, {}, [{'name': 'space_id', 'type': 'unsigned'},
       {'name': 'index_id', 'type': 'unsigned'}, {'name': 'func_id', 'type': 'unsigned'}]]
+  - [381, 1, '_vsession_settings', 'sysview', 0, {}, [{'name': 'name', 'type': 'string'},
+      {'name': 'value', 'type': 'any'}]]
 ...
 box.space._index:select{}
 ---
@@ -153,6 +155,7 @@ box.space._index:select{}
   - [364, 0, 'primary', 'tree', {'unique': true}, [[0, 'unsigned'], [1, 'string']]]
   - [372, 0, 'primary', 'tree', {'unique': true}, [[0, 'unsigned'], [1, 'unsigned']]]
   - [372, 1, 'fid', 'tree', {'unique': false}, [[2, 'unsigned']]]
+  - [381, 0, 'primary', 'tree', {'unique': true}, [[0, 'string']]]
 ...
 box.space._user:select{}
 ---
diff --git a/test/box/access_sysview.result b/test/box/access_sysview.result
index 1f33dec..c032248 100644
--- a/test/box/access_sysview.result
+++ b/test/box/access_sysview.result
@@ -246,11 +246,11 @@ box.session.su('guest')
 ...
 #box.space._vspace:select{}
 ---
-- 25
+- 26
 ...
 #box.space._vindex:select{}
 ---
-- 53
+- 54
 ...
 #box.space._vuser:select{}
 ---
@@ -282,7 +282,7 @@ box.session.su('guest')
 ...
 #box.space._vindex:select{}
 ---
-- 53
+- 54
 ...
 #box.space._vuser:select{}
 ---
@@ -743,3 +743,121 @@ s = box.space._space:create_index('owner', {parts = { 2, 'unsigned' }, id = 1, u
 session = nil
 ---
 ...
+--
+-- gh-4511: make sure that _vsession_settings sysview works as
+-- intended.
+--
+v = box.space._vsession_settings
+---
+...
+v:format()
+---
+- [{'name': 'name', 'type': 'string'}, {'name': 'value', 'type': 'any'}]
+...
+s = box.schema.space.create('settings', {format = v:format()})
+---
+...
+_ = s:create_index('primary')
+---
+...
+for _,value in v:pairs() do s:insert(value) end
+---
+...
+env = require('test_run')
+---
+...
+test_run = env.new()
+---
+...
+test_run:cmd('setopt delimiter ";"')
+---
+- true
+...
+function check_sorting(vs, ts, key)
+    local is_right = true
+    local iterators_list = {'ALL', 'REQ', 'EQ', 'GE', 'GT', 'LE', 'LT'}
+    for _, it in pairs(iterators_list) do
+        local view_space = vs:select({key}, {iterator = it})
+        local test_space = ts:select({key}, {iterator = it})
+        for key, value in pairs(view_space) do
+            is_right = is_right and (test_space[key].name == value.name)
+        end
+    end
+    return is_right
+end;
+---
+...
+test_run:cmd('setopt delimiter ""');
+---
+- true
+...
+check_sorting(v, s)
+---
+- true
+...
+check_sorting(v, s, 'abcde')
+---
+- true
+...
+check_sorting(v, s, 'sql_d')
+---
+- true
+...
+check_sorting(v, s, 'sql_v')
+---
+- true
+...
+check_sorting(v, s, 'sql_defer_foreign_keys')
+---
+- true
+...
+-- Check get() method.
+v:get({'sql_defer_foreign_keys'})
+---
+- ['sql_defer_foreign_keys', false]
+...
+v:get({'sql_recursive_triggers'})
+---
+- ['sql_recursive_triggers', true]
+...
+v:get({'sql_reverse_unordered_selects'})
+---
+- ['sql_reverse_unordered_selects', false]
+...
+v:get({'sql_default_engine'})
+---
+- ['sql_default_engine', 'memtx']
+...
+v:get({'abcd'})
+---
+...
+-- Make sure that space is read-only.
+new_record = v:frommap({name='abs', value=123})
+---
+...
+v:insert(new_record)
+---
+- error: View '_vsession_settings' is read-only
+...
+-- Check pairs() method.
+t = {}
+---
+...
+for key, value in v:pairs() do table.insert(t, {key, value}) end
+---
+...
+#t == v:count()
+---
+- true
+...
+-- Make sure that SQL SELECT works.
+box.execute([[SELECT * from "_vsession_settings" WHERE "name" = 'sql_default_engine']])
+---
+- metadata:
+  - name: name
+    type: string
+  - name: value
+    type: any
+  rows:
+  - ['sql_default_engine', 'memtx']
+...
diff --git a/test/box/access_sysview.test.lua b/test/box/access_sysview.test.lua
index 17420ae..a35a79c 100644
--- a/test/box/access_sysview.test.lua
+++ b/test/box/access_sysview.test.lua
@@ -308,3 +308,57 @@ s = box.space._space:create_index('owner', {parts = { 2, 'unsigned' }, id = 1, u
 #box.space._vspace.index[1]:select(1) > 0
 
 session = nil
+
+--
+-- gh-4511: make sure that _vsession_settings sysview works as
+-- intended.
+--
+
+v = box.space._vsession_settings
+v:format()
+
+s = box.schema.space.create('settings', {format = v:format()})
+_ = s:create_index('primary')
+for _,value in v:pairs() do s:insert(value) end
+
+env = require('test_run')
+test_run = env.new()
+test_run:cmd('setopt delimiter ";"')
+function check_sorting(vs, ts, key)
+    local is_right = true
+    local iterators_list = {'ALL', 'REQ', 'EQ', 'GE', 'GT', 'LE', 'LT'}
+    for _, it in pairs(iterators_list) do
+        local view_space = vs:select({key}, {iterator = it})
+        local test_space = ts:select({key}, {iterator = it})
+        for key, value in pairs(view_space) do
+            is_right = is_right and (test_space[key].name == value.name)
+        end
+    end
+    return is_right
+end;
+test_run:cmd('setopt delimiter ""');
+
+check_sorting(v, s)
+check_sorting(v, s, 'abcde')
+check_sorting(v, s, 'sql_d')
+check_sorting(v, s, 'sql_v')
+check_sorting(v, s, 'sql_defer_foreign_keys')
+
+-- Check get() method.
+v:get({'sql_defer_foreign_keys'})
+v:get({'sql_recursive_triggers'})
+v:get({'sql_reverse_unordered_selects'})
+v:get({'sql_default_engine'})
+v:get({'abcd'})
+
+-- Make sure that space is read-only.
+new_record = v:frommap({name='abs', value=123})
+v:insert(new_record)
+
+-- Check pairs() method.
+t = {}
+for key, value in v:pairs() do table.insert(t, {key, value}) end
+#t == v:count()
+
+-- Make sure that SQL SELECT works.
+box.execute([[SELECT * from "_vsession_settings" WHERE "name" = 'sql_default_engine']])
diff --git a/test/box/alter.result b/test/box/alter.result
index 46ce868..2a0eaca 100644
--- a/test/box/alter.result
+++ b/test/box/alter.result
@@ -92,7 +92,7 @@ space = box.space[t[1]]
 ...
 space.id
 ---
-- 373
+- 382
 ...
 space.field_count
 ---
@@ -137,7 +137,7 @@ space_deleted
 ...
 space:replace{0}
 ---
-- error: Space '373' does not exist
+- error: Space '382' does not exist
 ...
 _index:insert{_space.id, 0, 'primary', 'tree', {unique=true}, {{0, 'unsigned'}}}
 ---
@@ -220,6 +220,7 @@ _index:select{}
   - [364, 0, 'primary', 'tree', {'unique': true}, [[0, 'unsigned'], [1, 'string']]]
   - [372, 0, 'primary', 'tree', {'unique': true}, [[0, 'unsigned'], [1, 'unsigned']]]
   - [372, 1, 'fid', 'tree', {'unique': false}, [[2, 'unsigned']]]
+  - [381, 0, 'primary', 'tree', {'unique': true}, [[0, 'string']]]
 ...
 -- modify indexes of a system space
 _index:delete{_index.id, 0}
diff --git a/test/wal_off/alter.result b/test/wal_off/alter.result
index 62cb11d..97f7e6f 100644
--- a/test/wal_off/alter.result
+++ b/test/wal_off/alter.result
@@ -28,7 +28,7 @@ end;
 ...
 #spaces;
 ---
-- 65502
+- 65501
 ...
 -- cleanup
 for k, v in pairs(spaces) do
-- 
2.7.4