MDEV-17568: LATERAL DERIVED is not clearly visible in EXPLAIN FORMAT=JSON

Make LATERAL DERIVED tables visible in EXPLAIN FORMAT=JSON output.

mysql-test/main/derived_cond_pushdown.result

@@ -15115,6 +15115,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t1.a is not null",
@@ -15324,6 +15325,7 @@ EXPLAIN
       "filtered": 100,
       "attached_condition": "trigcond(trigcond(t1.a is not null))",
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t1.a is not null",
@@ -15418,6 +15420,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t3.a is not null and t3.c is not null",
@@ -15570,6 +15573,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t3.a is not null and t3.c is not null",
@@ -15742,6 +15746,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t3.c is not null",
@@ -15989,6 +15994,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "outer_ref_condition": "t3.c is not null",
@@ -16473,6 +16479,7 @@ EXPLAIN
       "filtered": 100,
       "first_match": "t4",
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 3,
           "const_condition": "1",
@@ -16619,6 +16626,7 @@ EXPLAIN
       "rows": 2,
       "filtered": 100,
       "materialized": {
+        "lateral": 1,
         "query_block": {
           "select_id": 2,
           "table": {

sql/sql_explain.cc

@@ -1756,6 +1756,11 @@ void Explain_table_access::print_explain_json(Explain_query *query,
     /* This is a derived table. Print its contents here */
     writer->add_member("materialized").start_object();
     Explain_node *node= query->get_node(derived_select_number);
+    if (node->get_type() == Explain_node::EXPLAIN_SELECT &&
+        ((Explain_select*)node)->is_lateral)
+    {
+      writer->add_member("lateral").add_ll(1);
+    }
     node->print_explain_json(query, writer, is_analyze);
     writer->end_object();
   }

sql/sql_explain.h

@@ -211,6 +211,7 @@ class Explain_select : public Explain_basic_join
     select_lex(NULL),
 #endif
     linkage(UNSPECIFIED_TYPE),
+    is_lateral(false),
     message(NULL),
     having(NULL), having_value(Item::COND_UNDEF),
     using_temporary(false), using_filesort(false),
@@ -226,6 +227,7 @@ class Explain_select : public Explain_basic_join
 #endif
   const char *select_type;
   enum sub_select_type linkage;
+  bool is_lateral;
 
   /*
     If message != NULL, this is a degenerate join plan, and all subsequent

sql/sql_select.cc

@@ -26014,6 +26014,8 @@ int JOIN::save_explain_data_intern(Explain_query *output,
     xpl_sel->select_id= join->select_lex->select_number;
     xpl_sel->select_type= join->select_lex->type;
     xpl_sel->linkage= select_lex->linkage;
+    xpl_sel->is_lateral= ((select_lex->linkage == DERIVED_TABLE_TYPE) &&
+                          (select_lex->uncacheable & UNCACHEABLE_DEPENDENT));
     if (select_lex->master_unit()->derived)
       xpl_sel->connection_type= Explain_node::EXPLAIN_NODE_DERIVED;