MDEV-7922 - ERROR 1939 (HY000): Engine PERFORMANCE_SCHEMA failed to discover

            table

Performance schema discovery fails if connection has no active database set.

This happened due to restriction in SQL parser: table name with no database name
is ambiguous in such case.

Fixed by temporary substitution of default database with being discovered table
database.

mysql-test/suite/perfschema/r/discovery.result

+#
+# MDEV-7922 - ERROR 1939 (HY000): Engine PERFORMANCE_SCHEMA failed to
+#             discover table
+#
+FLUSH TABLES;
+CREATE DATABASE test_drop;
+USE test_drop;
+DROP DATABASE test_drop;
+SELECT TABLE_NAME, TABLE_ROWS FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA='performance_schema';
+SELECT @@warning_count;
+@@warning_count
+0

mysql-test/suite/perfschema/t/discovery.test

+--source include/not_embedded.inc
+--source include/have_perfschema.inc
+
+--echo #
+--echo # MDEV-7922 - ERROR 1939 (HY000): Engine PERFORMANCE_SCHEMA failed to
+--echo #             discover table
+--echo #
+FLUSH TABLES;
+CREATE DATABASE test_drop;
+USE test_drop;
+DROP DATABASE test_drop;
+--disable_result_log
+SELECT TABLE_NAME, TABLE_ROWS FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA='performance_schema';
+--enable_result_log
+SELECT @@warning_count;

sql/table.cc

@@ -2158,6 +2158,7 @@ int TABLE_SHARE::init_from_sql_statement_string(THD *thd, bool write,
   uint unused2;
   handlerton *hton= plugin_hton(db_plugin);
   LEX_CUSTRING frm= {0,0};
+  LEX_STRING db_backup= { thd->db, thd->db_length };
 
   DBUG_ENTER("TABLE_SHARE::init_from_sql_statement_string");
 
@@ -2185,6 +2186,7 @@ int TABLE_SHARE::init_from_sql_statement_string(THD *thd, bool write,
   else
     thd->set_n_backup_active_arena(arena, &backup);
 
+  thd->reset_db(db.str, db.length);
   lex_start(thd);
 
   if ((error= parse_sql(thd, & parser_state, NULL) || 
@@ -2213,6 +2215,7 @@ int TABLE_SHARE::init_from_sql_statement_string(THD *thd, bool write,
 ret:
   my_free(const_cast<uchar*>(frm.str));
   lex_end(thd->lex);
+  thd->reset_db(db_backup.str, db_backup.length);
   thd->lex= old_lex;
   if (arena)
     thd->restore_active_arena(arena, &backup);