Fix optimizer_full_scan to not error for explain queries

Summary:
EXPLAIN queries with derived tables do not populate select_options in the JOIN structure properly. This means that we could error if we tried to run an explain on a query with derived tables.

Instead of reading from the JOIN struct, read from the LEX struct on the THD, as this is where the flag is original set on during query parsing.

This bug also means that in upstream, we increment some status variables tracking full table scans despite the fact that only an explain statement was done. This seems to have been fixed in 8.0 though.

Squash with: D7528820

Test Plan: extend test case

Reviewers: herman

Differential Revision: https://phabricator.intern.facebook.com/D7691148

Author: lth

Diff for: mysql-test/r/optimizer_full_scan.result

@@ -73,6 +73,12 @@ ERROR HY000: Full table/index scan is disabled
 set optimizer_full_scan = on;
 select * from t a straight_join t b where a.i = 10;
 i	j	i	j
+set optimizer_full_scan = off;
+explain select * from (select * from t) a group by i;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	6	Using temporary; Using filesort
+2	DERIVED	t	ALL	NULL	NULL	NULL	NULL	6	NULL
+set optimizer_full_scan = on;
 # Test integration with optimizer_force_index_for_range
 alter table t drop index i, add primary key (i, j);
 # Test range plans

Diff for: mysql-test/t/optimizer_full_scan.test

@@ -44,6 +44,11 @@ select * from t a straight_join t b where a.i = 10;
 set optimizer_full_scan = on;
 select * from t a straight_join t b where a.i = 10;
 
+--replace_column 9 #
+set optimizer_full_scan = off;
+explain select * from (select * from t) a group by i;
+set optimizer_full_scan = on;
+
 --echo # Test integration with optimizer_force_index_for_range
 alter table t drop index i, add primary key (i, j);
 

Diff for: sql/sql_select.cc

@@ -2809,7 +2809,7 @@ bool JOIN::setup_materialized_table(JOIN_TAB *tab, uint tableno,
 bool
 make_join_readinfo(JOIN *join, ulonglong options, uint no_jbuf_after)
 {
-  const bool statistics= MY_TEST(!(join->select_options & SELECT_DESCRIBE));
+  const bool statistics= MY_TEST(!(join->thd->lex->select_lex.options & SELECT_DESCRIBE));
 
   DBUG_ENTER("make_join_readinfo");
 
@@ -2900,12 +2900,14 @@ make_join_readinfo(JOIN *join, ulonglong options, uint no_jbuf_after)
 	  {
 	    join->thd->set_status_no_index_used();
 	    if (statistics)
+	    {
 	      join->thd->inc_status_select_scan();
-	    /* Block full table/index scans, if optimizer_full_scan is off. */
-	    if (!join->thd->variables.optimizer_full_scan &&
-	       !(join->select_options & SELECT_DESCRIBE)) {
-	      my_error(ER_FULL_SCAN_DISABLED, MYF(0));
-	      DBUG_RETURN(TRUE);
+	      /* Block full table/index scans, if optimizer_full_scan is off. */
+	      if (!join->thd->variables.optimizer_full_scan)
+	      {
+	        my_error(ER_FULL_SCAN_DISABLED, MYF(0));
+	        DBUG_RETURN(TRUE);
+	      }
 	    }
 	  }
 	}
@@ -2920,12 +2922,14 @@ make_join_readinfo(JOIN *join, ulonglong options, uint no_jbuf_after)
 	  {
 	    join->thd->set_status_no_index_used();
 	    if (statistics)
+	    {
 	      join->thd->inc_status_select_full_join();
-	    /* Block full table/index scans, if optimizer_full_scan is off. */
-	    if (!join->thd->variables.optimizer_full_scan &&
-	       !(join->select_options & SELECT_DESCRIBE)) {
-	      my_error(ER_FULL_SCAN_DISABLED, MYF(0));
-	      DBUG_RETURN(TRUE);
+	      /* Block full table/index scans, if optimizer_full_scan is off. */
+	      if (!join->thd->variables.optimizer_full_scan)
+	      {
+	        my_error(ER_FULL_SCAN_DISABLED, MYF(0));
+	        DBUG_RETURN(TRUE);
+	      }
 	    }
 	  }
 	}