Re: [Tarantool-patches] [tarantool-patches] Re: [PATCH v2 2/5] box: introdice _vsession_settings sysview

[Mergen Imeev <imeevma@tarantool.org>]

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

On Sat, Oct 26, 2019 at 07:35:27PM +0200, Vladislav Shpilevoy wrote:
> Hi! Thanks for the fixes!
> 
> On 25/10/2019 17:45, imeevma@tarantool.org wrote:
> > Thank you for review! My answers, one question and new patch
> > below.
> > 
> > On 10/19/19 1:08 AM, Vladislav Shpilevoy wrote:
> >> Thanks for the patch!
> >>
> >> See 14 comments below.
> >>
> >> On 17/10/2019 16:40, imeevma@tarantool.org wrote:
> >>> This patch creates the _vsql_settings sysview. This system view
> >>> has custom methods get() and create_iterator(). This allows us
> >>> to get SQL settings by creating a tuple on the fly without having
> >>> to save it anywhere.
> >>>
> >>> Part of #4511
> >>>
> >>> @TarantoolBot document
> >>
> >> 1. I think, it is worth to create one doc request for the
> >> whole feature - both SET command and _vsql_settings space.
> >> They are useless without each other.
> >>
> > Fixed. However, do I need to write a doc-bot request now when the
> > sysview purpose has changed?
> 
> Nah, I still think one request for the whole _vsession_settings
> feature is enough. You describe what is _vsession_settings,
> which options it has already, how to set them, and it should be
> sufficient.
> 
> > diff --git a/src/box/session.cc b/src/box/session.cc
> > index 59bf226..32dc87a 100644
> > --- a/src/box/session.cc
> > +++ b/src/box/session.cc
> 
> Recently I've seen a ticket https://github.com/tarantool/tarantool/issues/4205
> about read-only sessions. That looks like a first candidate on a
> non-SQL session local setting.
> 
> That means we probably need to design how to make session
> settings iterator more generic. Currently all the settings
> iteration code consists of explicit SQL things.
> 
> On the other hand I don't really know whether we should
> consider read-only option a session setting.
> And do we need to create a more generic session settings
> iterator in scope of this patchset?
> 
> Here is my personal opinion how it should be done:
> 
>     - We push this patchset with current SQL-centric design of
>       the settings iteration to stabilize the public API as
>       soon as possible;
> 
>     - Later in scope of 4205 we add session-local option
>       read-only and reimplement session settings iterator so
>       as it would not depend on SQL so much.
> 
> 
> See 6 comments below.
> 
> > @@ -360,3 +363,116 @@ generic_session_sync(struct session *session)
> > +
> > +struct iterator *
> > +session_options_create_iterator(struct index *index, enum iterator_type type,
> > +				const char *key, uint32_t part_count)
> > +{
> > +	int max_tuples_to_show = -1;
> > +	uint32_t sql_option_id;
> > +	if (part_count > 0) {
> > +		assert(part_count == 1);
> > +		uint32_t len;
> > +		const char *name = mp_decode_str(&key, &len);
> > +		name = tt_cstr(name, len);
> > +		sql_option_id = sql_option_id_by_name(name);
> > +		if (type == ITER_EQ || type == ITER_REQ)
> > +			max_tuples_to_show = 1;
> > +		else if (type == ITER_LT)
> > +			--sql_option_id;
> > +		else if (type == ITER_GT)
> > +			++sql_option_id;
> > +		if (sql_option_id == sql_option_id_max() && type == ITER_LE)
> > +			--sql_option_id;
> 
> 1. Strangely GE does not return anything here:
> 
> tarantool> box.space._vsession_settings:select({'sql_'}, {iterator = 'GE'})
> ---
> - []
> ...
> 
> LE also behaves strange:
> 
> tarantool> box.space._vsession_settings:select({'sql_d'}, {iterator = 'LE'})
> ---
> - - ['sql_where_trace', false]
>   - ['sql_vdbe_trace', false]
>   - ['sql_vdbe_listing', false]
>   - ['sql_vdbe_eqp', false]
>   - ['sql_vdbe_debug', false]
>   - ['sql_vdbe_addoptrace', false]
>   - ['sql_trace', false]
>   - ['sql_select_trace', false]
>   - ['sql_reverse_unordered_selects', false]
>   - ['sql_recursive_triggers', true]
>   - ['sql_parser_trace', false]
>   - ['sql_full_column_names', false]
>   - ['sql_defer_foreign_keys', false]
>   - ['sql_default_engine', 'memtx']
>   - ['sql_compound_select_limit', 30]
> ...
> 
> 'sql_where_trace' definitely is not <= 'sql_d'.
> 
> This is why I was thinking that probably our _vsession_settings
> iterator API should be very limited. After all, it is a fixed
> size space with settings. A user don't need to be able to dance
> with it using different iterators. I would allow only EQ, and either
> empty key, or a subsystem name like 'sql', or a full key for a
> concrete setting.
> 
> Up to you. And probably to Kirill since it is a public API.
> 
I fixed behaviour of iterators.

> > +	} else {
> > +		sql_option_id = iterator_type_is_reverse(type) ?
> > +				sql_option_id_max() - 1 : 0;
> > +	}
> > diff --git a/src/box/sql/build.c b/src/box/sql/build.c
> > index 51cd7ce..cb97106 100644
> > --- a/src/box/sql/build.c
> > +++ b/src/box/sql/build.c
> > @@ -3242,3 +3243,160 @@ sql_fieldno_by_name(struct Parse *parse_context, struct Expr *field_name,
> > +/**
> > + * 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.
> > + */
> 
> 2. Please, add a test that selects all the options and checks
> that they are sorted. It would prevent accidental reorders.
> 
Not sure that we can do this since some of the options
appears only in debug build. But I reworked some tests
using additional space to check sorting and iterator.

> > +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;
> 
> 3. What is the option mask?
> 
Previously it was "flag", but since one of the options
(sql_debug) have three three of flags to set/unset I
changed its name to "mask".

> > +};
> > +
> > +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 region *region = &fiber()->gc;
> > +	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];
> 
> 4. So the SQL_LIMIT_COMPOUND_SELECT is not session local? Then
> why is it in the session settings?
> 
True, I forgot about this when changed sysview. Still, I do
not know where should we move this option.

> > +		size += mp_sizeof_uint(limit);
> > +	}
> > +
> > +	size_t svp = region_used(region);
> > +	char *pos_ret = region_alloc(region, size);
> 
> 5. Up to you, but probably static_alloc() would work well here.
> Size of option tuples is limited, and static_alloc() returns up
> to 12KB that should be more than enough here. We could assert,
> that static_alloc() returns not NULL for the given size.
> 
Thanks, fixed.

> > +	if (pos_ret == NULL) {
> > +		diag_set(OutOfMemory, size, "region_alloc", "pos_ret");
> > +		return -1;
> > +	}
> > +	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);
> > +	region_truncate(region, svp);
> > +	if (tuple == NULL)
> > +		return -1;
> > +	*result = tuple;
> > +	return 0;
> > +}
> > diff --git a/test/box/misc.result b/test/box/misc.result
> > index b293051..b41ffa4 100644
> > --- a/test/box/misc.result
> > +++ b/test/box/misc.result
> > @@ -1330,3 +1330,124 @@ test_run:grep_log('default', 'test log')
> >  ---
> >  - test log
> >  ...
> > +--
> > +-- gh-4511: make sure that _vsession_settings sysview works as
> > +-- intended.
> > +--
> > +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
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'ALL'})) == 0
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'REQ'})) == 0
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'EQ'})) == 0
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'GE'})) == 0
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'GT'})) == 0
> > +---
> > +- true
> > +...
> > +(#v:select({'abcde'}, {iterator = 'LE'})) == option_count
> > +---
> > +- true
> > +...
> 
> 6. Are you sure? We have no options, whose name is <= 'a'.
> At least because all of them start with 'sql_' which > 'a'.
> 
> If you want to keep these iterator types, then probably
> the most correct test would fill a normal space with
> session settings, and check for each of these keys above,
> that the virtual space and the real one return the same.
> 
Thank you for suggestion! I reworked tests using additional
space.


New patch:

From 61f75f2b4a800bab37ba5195b71c488fe0d145ee 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 d69b657..6d301bf 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 +364,135 @@ 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;
+		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);
+		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/session.h b/src/box/session.h
index 85a2d94..96d9ad0 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
@@ -355,6 +358,16 @@ 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..ad85f1e 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 +421,40 @@ 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 option.
+ *
+ * @retval Next number after the last SQL option.
+ */
+uint32_t
+sql_option_id_max();
+
+/**
+ * Return an integer less than, equal to, or greater than zero if
+ * name of 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_option_compare(const char *key, uint32_t id);
+
+/**
+ * Create a tuple that contains the name and value of the SQL
+ * 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_option_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..dceb092 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 +3243,151 @@ 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);
+	}
+
+	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/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 3072b73..5a41526 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{}
 ---
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/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.
+--
+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
+---
+- 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
+...
+v:select({'sql_defer_foreign_keys'})
+---
+- - ['sql_defer_foreign_keys', false]
+...
+v:select({'sql_recursive_triggers'})
+---
+- - ['sql_recursive_triggers', true]
+...
+v:select({'sql_reverse_unordered_selects'})
+---
+- - ['sql_reverse_unordered_selects', false]
+...
+v:select({'sql_compound_select_limit'})
+---
+- - ['sql_compound_select_limit', 30]
+...
+v:select({'sql_default_engine'})
+---
+- - ['sql_default_engine', 'memtx']
+...
+-- 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_compound_select_limit'})
+---
+- ['sql_compound_select_limit', 30]
+...
+v:get({'sql_default_engine'})
+---
+- ['sql_default_engine', 'memtx']
+...
+-- 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 box.space._vsession_settings:pairs() do table.insert(t, {key, value}) end
+---
+...
+#t == option_count
+---
+- true
+...
+-- Check sorting.
+in_sysview = v:select()
+---
+...
+in_space = s:select()
+---
+...
+is_right = true
+---
+...
+for key, value in pairs(in_sysview) do is_right = is_right and (in_space[key].name == value.name) end
+---
+...
+is_right
+---
+- 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/misc.test.lua b/test/box/misc.test.lua
index cc223b2..037c866 100644
--- a/test/box/misc.test.lua
+++ b/test/box/misc.test.lua
@@ -383,3 +383,66 @@ tuple_format = box.internal.new_tuple_format(format)
 box.cfg{}
 local ffi = require'ffi' ffi.C._say(ffi.C.S_WARN, nil, 0, nil, "%s", "test log")
 test_run:grep_log('default', 'test log')
+
+--
+-- gh-4511: make sure that _vsession_settings sysview works as
+-- intended.
+--
+
+v = box.space._vsession_settings
+option_count = v:count()
+
+v:format()
+
+(#v:select()) == option_count
+(#v:select({}, {iterator = 'ALL'})) == option_count
+(#v:select({}, {iterator = 'REQ'})) == option_count
+(#v:select({}, {iterator = 'EQ'})) == option_count
+(#v:select({}, {iterator = 'GE'})) == option_count
+(#v:select({}, {iterator = 'GT'})) == option_count
+(#v:select({}, {iterator = 'LE'})) == option_count
+(#v:select({}, {iterator = 'LT'})) == option_count
+
+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'}))
+(#v:select({'abcde'}, {iterator = 'REQ'})) == (#s:select({'abcde'}, {iterator = 'REQ'}))
+(#v:select({'abcde'}, {iterator = 'EQ'})) == (#s:select({'abcde'}, {iterator = 'EQ'}))
+(#v:select({'abcde'}, {iterator = 'GE'})) == (#s:select({'abcde'}, {iterator = 'GE'}))
+(#v:select({'abcde'}, {iterator = 'GT'})) == (#s:select({'abcde'}, {iterator = 'GT'}))
+(#v:select({'abcde'}, {iterator = 'LE'})) == (#s:select({'abcde'}, {iterator = 'LE'}))
+(#v:select({'abcde'}, {iterator = 'LT'})) == (#s:select({'abcde'}, {iterator = 'LT'}))
+
+v:select({'sql_defer_foreign_keys'})
+v:select({'sql_recursive_triggers'})
+v:select({'sql_reverse_unordered_selects'})
+v:select({'sql_compound_select_limit'})
+v:select({'sql_default_engine'})
+
+-- Check get() method.
+v:get({'sql_defer_foreign_keys'})
+v:get({'sql_recursive_triggers'})
+v:get({'sql_reverse_unordered_selects'})
+v:get({'sql_compound_select_limit'})
+v:get({'sql_default_engine'})
+
+-- 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 box.space._vsession_settings:pairs() do table.insert(t, {key, value}) end
+#t == option_count
+
+-- Check sorting.
+in_sysview = v:select()
+in_space = s:select()
+is_right = true
+for key, value in pairs(in_sysview) do is_right = is_right and (in_space[key].name == value.name) end
+is_right
+
+-- Make sure that SQL SELECT works.
+box.execute([[SELECT * from "_vsession_settings" WHERE "name" = 'sql_default_engine']])
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