sql: remove one row limit in EXISTS subquery

According to ANSI, EXISTS is a predicate that tests a given subquery and
returns true if it returns more than 0 rows, false otherwise. However,
before this patch, EXISTS worked correctly only if there were exactly 0
or 1 rows, and in all other cases it gave an error. This patch makes
EXITS work properly.

Closes tarantool#8676

@TarantoolBot document
Title: EXISTS predicate

The EXISTS predicate in SQL is a predicate that returns true if the
given SQL subquery returns at least one row, and false otherwise.

Example:
```
SELECT EXISTS(SELECT * FROM (VALUES(1), (2)));
```

changelogs/unreleased/gh-8676-fix-exists-predicate.md

@@ -0,0 +1,4 @@
+## bugfix/sql
+
+* The `EXIST` predicate now returns true if the subquery returns more than 0
+  rows, and false otherwise (gh-8676).

src/box/sql/expr.c

@@ -2801,7 +2801,8 @@ sqlCodeSubselect(Parse * pParse,	/* Parsing context */
 							    &sqlIntTokens[1]);
 				ExprSetProperty(pSel->pLimit, EP_System);
 			}
-			pSel->selFlags |= SF_SingleRow;
+			if (pExpr->op == TK_SELECT)
+				pSel->selFlags |= SF_SingleRow;
 			pSel->iLimit = 0;
 			pSel->selFlags &= ~SF_MultiValue;
 			if (sqlSelect(pParse, pSel, &dest)) {

test/sql-luatest/gh_8676_fix_exists_test.lua

@@ -0,0 +1,20 @@
+local server = require('luatest.server')
+local t = require('luatest')
+
+local g = t.group()
+
+g.before_all(function()
+    g.server = server:new({alias = 'master'})
+    g.server:start()
+end)
+
+g.after_all(function()
+    g.server:stop()
+end)
+
+g.test_exists = function()
+    g.server:exec(function()
+        local res = box.execute([[SELECT EXISTS (VALUES (1), (2));]])
+        t.assert_equals(res.rows, {{true}})
+    end)
+end

test/sql-tap/tkt1473.test.lua

@@ -199,14 +199,14 @@ test:do_execsql_test(
         -- </tkt1473-2.9>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-3.2",
     [[
         SELECT EXISTS
           (SELECT 1 FROM t1 WHERE a=1 UNION ALL SELECT 2 FROM t1 WHERE b=0)
     ]], {
         -- <tkt1473-3.2>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-3.2>
     })
 
@@ -221,36 +221,36 @@ test:do_execsql_test(
         -- </tkt1473-3.3>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-3.4",
     [[
         SELECT EXISTS
           (SELECT 1 FROM t1 WHERE a=1 UNION ALL SELECT 2 FROM t1 WHERE b=4)
     ]], {
         -- <tkt1473-3.4>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-3.4>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-3.5",
     [[
         SELECT EXISTS
           (SELECT 1 FROM t1 WHERE a=1 UNION SELECT 2 FROM t1 WHERE b=4)
     ]], {
         -- <tkt1473-3.5>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-3.5>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-3.6",
     [[
         SELECT EXISTS
           (SELECT 1 FROM t1 WHERE a=0 UNION ALL SELECT 2 FROM t1 WHERE b=4)
     ]], {
         -- <tkt1473-3.6>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-3.6>
     })
 
@@ -483,7 +483,7 @@ test:do_execsql_test(
         -- </tkt1473-4.7>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-5.3",
     [[
         SELECT EXISTS (
@@ -509,11 +509,11 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-5.3>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-5.3>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-5.4",
     [[
         SELECT EXISTS (
@@ -539,11 +539,11 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-5.4>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-5.4>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-5.5",
     [[
         SELECT EXISTS (
@@ -569,11 +569,11 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-5.5>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-5.5>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-5.6",
     [[
         SELECT EXISTS (
@@ -599,7 +599,7 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-5.6>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-5.6>
     })
 
@@ -633,7 +633,7 @@ test:do_execsql_test(
         -- </tkt1473-5.7>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-6.3",
     [[
         SELECT EXISTS (
@@ -659,11 +659,11 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-6.3>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-6.3>
     })
 
-test:do_catchsql_test(
+test:do_execsql_test(
     "tkt1473-6.4",
     [[
         SELECT EXISTS (
@@ -689,7 +689,7 @@ test:do_catchsql_test(
         )
     ]], {
         -- <tkt1473-6.4>
-        1, "Failed to execute SQL statement: Expression subquery returned more than 1 row"
+        true
         -- </tkt1473-6.4>
     })