Hi! Thank you for review! Diff between versions and new patch
below.

Please, don’t state problem in commit message. Instead, it would be
better to indicate fix of the problem. I would call it like:

sql: decode mgspack after SELECT from system spaces

Otherwise it looks like you are introducing new bug :)

Also, your patch affects not only system spaces.
For example:

box.schema.space.create('t’)
box.space.t:format({{name = 'id', type = 'integer'}, {name = 'x', type = 'any'}})
box.space.t:create_index('i1', {parts = {1, 'int'}})
box.space.t:insert({1, {1,2,3}})
box.space.t:insert({2, {a = 3}})

Before your patch:

tarantool> cn:execute('select * from \"t\"')
---
- metadata:
  - name: id
    type: UNKNOWN
  - name: x
    type: UNKNOWN
  rows:
  - [1, !!binary kwECAw==]
  - [2, !!binary gaFhAw==]
...

After:

tarantool> cn:execute('select * from \"t\"')
---
- metadata:
  - name: id
    type: UNKNOWN
  - name: x
    type: UNKNOWN
  rows:
  - [1, [1, 2, 3]]
  - [2, {'a': 3}]
…

I guess more precise to say sort of:

sql: decode ARRAY and MAP types after SELECT

Patch itself is OK, but I would rework test in order to avoid
using system spaces. It is up to you.

Diff between versions:

diff --git a/test/sql/iproto.result b/test/sql/iproto.result
index b866140..6c50781 100644
--- a/test/sql/iproto.result
+++ b/test/sql/iproto.result
@@ -811,14 +811,37 @@ cn:execute("PRAGMA TABLE_INFO(test);")
 box.sql.execute('DROP TABLE test')
 ---
 ...
--- SELECT from system spaces returns unpacked msgpack.
-res = cn:execute('select "format" from "_space" limit 1;')
+-- SELECT returns unpacked msgpack.
+format = {{name = 'id', type = 'integer'}, {name = 'x', type = 'any'}}
 ---
 ...
-res.rows
+s = box.schema.space.create('test', {format=format})
+---
+...
+i1 = s:create_index('i1', {parts = {1, 'int'}})
+---
+...
+s:insert({1, {1,2,3}})
+---
+- [1, [1, 2, 3]]
+...
+s:insert({2, {a = 3}})
+---
+- [2, {'a': 3}]
+...
+cn:execute('select * from "test"')
+---
+- metadata:
+  - name: id
+    type: UNKNOWN
+  - name: x
+    type: UNKNOWN
+  rows:
+  - [1, [1, 2, 3]]
+  - [2, {'a': 3}]
+...
+s:drop()
 ---
-- - [[{'name': 'space_id', 'type': 'unsigned'}, {'name': 'lsn', 'type': 'unsigned'},
-      {'name': 'tuple', 'type': 'array'}]]
 ...
 cn:close()
 ---
diff --git a/test/sql/iproto.test.lua b/test/sql/iproto.test.lua
index 6299814..e12decd 100644
--- a/test/sql/iproto.test.lua
+++ b/test/sql/iproto.test.lua
@@ -263,9 +263,14 @@ box.sql.execute('CREATE TABLE test (id INT PRIMARY KEY)')
 cn:execute("PRAGMA TABLE_INFO(test);")
 box.sql.execute('DROP TABLE test')
 
--- SELECT from system spaces returns unpacked msgpack.
-res = cn:execute('select "format" from "_space" limit 1;')
-res.rows
+-- SELECT returns unpacked msgpack.
+format = {{name = 'id', type = 'integer'}, {name = 'x', type = 'any'}}
+s = box.schema.space.create('test', {format=format})
+i1 = s:create_index('i1', {parts = {1, 'int'}})
+s:insert({1, {1,2,3}})
+s:insert({2, {a = 3}})
+cn:execute('select * from "test"')
+s:drop()
 
 cn:close()
 
New patch:

commit 8482e0c0bc1fa44faa6abf7b7b8a27b77e91db92
Author: Mergen Imeev <imeevma@gmail.com>
Date:   Fri Nov 16 14:23:39 2018 +0300

    sql: decode ARRAY and MAP types after SELECT
   
    Before this patch MSGPACK received using SELECT statement through
    net.box was unpacked. Fixed in this patch.

diff --git a/src/box/execute.c b/src/box/execute.c
index cd809f8..fb3e08b 100644
--- a/src/box/execute.c
+++ b/src/box/execute.c
@@ -351,13 +351,21 @@ sql_column_to_messagepack(struct sqlite3_stmt *stmt, int i,
     }
     case SQLITE_BLOB: {
         uint32_t len = sqlite3_column_bytes(stmt, i);
-        size = mp_sizeof_bin(len);
-        char *pos = (char *) region_alloc(region, size);
-        if (pos == NULL)
-            goto oom;
-        const char *s;
-        s = (const char *)sqlite3_column_blob(stmt, i);
-        mp_encode_bin(pos, s, len);
+        const char *s =
+            (const char *)sqlite3_column_blob(stmt, i);
+        if (sql_column_subtype(stmt, i) == SQL_SUBTYPE_MSGPACK) {
+            size = len;
+            char *pos = (char *)region_alloc(region, size);
+            if (pos == NULL)
+                goto oom;
+            memcpy(pos, s, len);
+        } else {
+            size = mp_sizeof_bin(len);
+            char *pos = (char *)region_alloc(region, size);
+            if (pos == NULL)
+                goto oom;
+            mp_encode_bin(pos, s, len);
+        }
         break;
     }
     case SQLITE_NULL: {
diff --git a/test/sql/iproto.result b/test/sql/iproto.result
index f229427..6c50781 100644
--- a/test/sql/iproto.result
+++ b/test/sql/iproto.result
@@ -811,6 +811,38 @@ cn:execute("PRAGMA TABLE_INFO(test);")
 box.sql.execute('DROP TABLE test')
 ---
 ...
+-- SELECT returns unpacked msgpack.
+format = {{name = 'id', type = 'integer'}, {name = 'x', type = 'any'}}
+---
+...
+s = box.schema.space.create('test', {format=format})
+---
+...
+i1 = s:create_index('i1', {parts = {1, 'int'}})
+---
+...
+s:insert({1, {1,2,3}})
+---
+- [1, [1, 2, 3]]
+...
+s:insert({2, {a = 3}})
+---
+- [2, {'a': 3}]
+...
+cn:execute('select * from "test"')
+---
+- metadata:
+  - name: id
+    type: UNKNOWN
+  - name: x
+    type: UNKNOWN
+  rows:
+  - [1, [1, 2, 3]]
+  - [2, {'a': 3}]
+...
+s:drop()
+---
+...
 cn:close()
 ---
 ...
diff --git a/test/sql/iproto.test.lua b/test/sql/iproto.test.lua
index 9bcc7ef..e12decd 100644
--- a/test/sql/iproto.test.lua
+++ b/test/sql/iproto.test.lua
@@ -263,6 +263,15 @@ box.sql.execute('CREATE TABLE test (id INT PRIMARY KEY)')
 cn:execute("PRAGMA TABLE_INFO(test);")
 box.sql.execute('DROP TABLE test')
 
+-- SELECT returns unpacked msgpack.
+format = {{name = 'id', type = 'integer'}, {name = 'x', type = 'any'}}
+s = box.schema.space.create('test', {format=format})
+i1 = s:create_index('i1', {parts = {1, 'int'}})
+s:insert({1, {1,2,3}})
+s:insert({2, {a = 3}})
+cn:execute('select * from "test"')
+s:drop()
+
 cn:close()
 
 box.schema.user.revoke('guest', 'read,write,execute', 'universe')