sql: Limit number of terms in SELECT compoind stmts

Right now SQL query compiler is run on top of fiber's stack,
which is limited to 64KB by default, so maximum
number of entities in compound SELECT statement should be less than
50 (verified by experiment) or stack guard will be triggered.

In future we'll introduce heuristic which should understand that
query is 'complex' and run compilation in separate thread with larger
stack. This will also allow us not to block Tarantool while compiling
the query.

Closes #2548

src/box/sql/sqliteLimit.h

@@ -77,9 +77,13 @@
 ** if the number of terms is too large.  In practice, most SQL
 ** never has more than 3 or 4 terms.  Use a value of 0 to disable
 ** any limit on the number of terms in a compount SELECT.
+**
+** Tarantool: gh-2548: Fiber stack is 64KB by default, so maximum
+** number of entities should be less than 50 or stack guard will be
+** triggered.
 */
 #ifndef SQLITE_MAX_COMPOUND_SELECT
-# define SQLITE_MAX_COMPOUND_SELECT 500
+# define SQLITE_MAX_COMPOUND_SELECT 50
 #endif
 
 /*

test/sql/sql-select-compound-limit.lua

+
+box.cfg{wal_mode='none'}
+
+table_count = 51
+
+for _, term in ipairs({'UNION', 'UNION ALL', 'INTERSECT', 'EXCEPT'}) do 
+    for i = 1,table_count do
+        drop_string = 'DROP TABLE IF EXISTS t' .. i .. ';\n'
+        box.sql.execute(drop_string)
+    end
+
+    for i = 1,table_count do
+        create_string = 'CREATE TABLE t' .. i .. ' (s1 int primary key, s2 int);\n'
+        box.sql.execute(create_string)
+    end
+
+    for i = 1,table_count do
+        insert_string = 'INSERT INTO t' .. i .. ' VALUES (0,' .. i .. ');\n'
+        box.sql.execute(insert_string)
+    end
+
+    select_string = ''
+    for i = 1,table_count-1 do
+        if i > 1 then select_string = select_string .. ' ' .. term .. ' ' end
+        select_string = select_string .. 'SELECT * FROM t' .. i
+    end
+
+    if not pcall(function() box.sql.execute(select_string) end) then
+        print('not ok')
+    end
+
+    select_string = select_string .. ' ' .. term ..' ' .. 'SELECT * FROM t' .. table_count
+    if pcall(function() box.sql.execute(select_string) end) then
+        print('not ok')
+    end
+end
+