MDEV-3798: EXPLAIN UPDATE/DELETE:

Backport mysql-test/t/myisam_explain_non_select_all.test from mysql-5.6
- the .result file was modified because MariaDB choses different 
  query plans in a number cases. Also, we don't have some of the 
  "incorrect EXPLAIN output" bugs that they still have.

The .test file and includes were taken verbatim with one 
exception: two tests were disabled with --disable parsing:

1. @@sql_safe_updates is not enforced EXPLAINs of multitable 
   updates. In MariaDB, the execution itself will produce 
   ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE, but EXPLAIN won't.
2. Their case #71 hits some old bug in MyISAM (See their comments
   in explain_non_select.inc for details).

mysql-test/include/explain_utils.inc

+# This file is a collection of utility tests
+# for WL#4897: Add EXPLAIN INSERT/UPDATE/DELETE.
+#
+# Since MTR doesn't have functions, we use this file instead
+# including it many times.
+#
+# Parameters:
+#
+#  $query:   INSERT/REPLACE/UPDATE/DELETE query to explain
+#            NOTE: this file resets this variable
+#
+#  $select:  may be empty; the SELECT query similar to $query
+#            We use it to compare:
+#              1) table data before and after EXPLAIN $query evaluation;
+#              2) EXPLAIN $query and EXPLAIN $select output and
+#                 handler/filesort statistics
+#            NOTE: this file resets this variable
+#  $innodb:  take $no_rows parameter into account if not 0;
+#  $no_rows: filter out "rows" and "filtered" columns of EXPLAIN if not 0;
+#            it may be necessary for InnoDB tables since InnoDB's table row
+#            counter can't return precise and repeatable values;
+#	     NOTE: ANALYZE TABLE doesn't help
+#            NOTE: this file resets this variable
+
+--echo #
+--echo # query:  $query
+--echo # select: $select
+--echo #
+
+if ($select) {
+--disable_query_log
+--eval $select INTO OUTFILE '$MYSQLTEST_VARDIR/tmp/before_explain.txt'
+--enable_query_log
+}
+
+if ($innodb) {
+  if ($no_rows) {
+--replace_column 9 X
+  }
+}
+--eval EXPLAIN $query
+if (`SELECT ROW_COUNT() > 0`) {
+--echo # Erroneous query: EXPLAIN $query
+--die Unexpected ROW_COUNT() <> 0
+}
+
+FLUSH STATUS;
+FLUSH TABLES;
+if ($innodb) {
+  if ($no_rows) {
+--replace_column 9 X 10 X
+  }
+}
+--eval EXPLAIN EXTENDED $query
+if (`SELECT ROW_COUNT() > 0`) {
+--echo # Erroneous query: EXPLAIN EXTENDED $query
+--die Unexpected ROW_COUNT() <> 0
+}
+--echo # Status of EXPLAIN EXTENDED query
+--disable_query_log
+SHOW STATUS WHERE (Variable_name LIKE 'Sort%' OR
+                   Variable_name LIKE 'Handler_read_%' OR
+                   Variable_name = 'Handler_write' OR
+                   Variable_name = 'Handler_update' OR
+                   Variable_name = 'Handler_delete') AND Value <> 0;
+--enable_query_log
+
+if ($json) {
+if ($innodb) {
+  if ($no_rows) {
+--replace_regex /"rows": [0-9]+/"rows": "X"/ /"filtered": [0-9.]+/"filtered": "X"/
+  }
+}
+--eval EXPLAIN FORMAT=JSON $query;
+if ($validation) {
+--disable_query_log
+--replace_result $MASTER_MYSOCK MASTER_MYSOCK
+--exec $MYSQL -S $MASTER_MYSOCK -u root -r test -e "EXPLAIN FORMAT=JSON $query;" > $MYSQLTEST_VARDIR/tmp/explain.json
+--replace_regex  /[-]*// /FILE.[\/\\:_\.0-9A-Za-z]*/Validation:/
+--exec python $MYSQL_TEST_DIR/suite/opt_trace/validate_json.py $MYSQLTEST_VARDIR/tmp/explain.json 
+--remove_file '$MYSQLTEST_VARDIR/tmp/explain.json'
+--enable_query_log
+}
+}
+
+if ($select) {
+FLUSH STATUS;
+FLUSH TABLES;
+if ($innodb) {
+  if ($no_rows) {
+--replace_column 9 X 10 X
+  }
+}
+--eval EXPLAIN EXTENDED $select
+--echo # Status of EXPLAIN EXTENDED "equivalent" SELECT query execution
+--disable_query_log
+SHOW STATUS WHERE (Variable_name LIKE 'Sort%' OR
+                   Variable_name LIKE 'Handler_read_%' OR
+                   Variable_name = 'Handler_write' OR
+                   Variable_name = 'Handler_update' OR
+                   Variable_name = 'Handler_delete') AND Value <> 0;
+--enable_query_log
+if ($json) {
+if ($innodb) {
+  if ($no_rows) {
+--replace_regex /"rows": [0-9]+/"rows": "X"/ /"filtered": [0-9.]+/"filtered": "X"/
+  }
+}
+--eval EXPLAIN FORMAT=JSON $select;
+if ($validation) {
+--disable_query_log
+--replace_result $MASTER_MYSOCK MASTER_MYSOCK
+--exec $MYSQL -S $MASTER_MYSOCK -u root -r test -e "EXPLAIN FORMAT=JSON $select;" > $MYSQLTEST_VARDIR/tmp/explain.json
+--replace_regex  /[-]*// /FILE.[\/\\:_\.0-9A-Za-z]*/Validation:/
+--exec python $MYSQL_TEST_DIR/suite/opt_trace/validate_json.py $MYSQLTEST_VARDIR/tmp/explain.json 
+--remove_file '$MYSQLTEST_VARDIR/tmp/explain.json'
+--enable_query_log
+}
+}
+}
+
+--disable_query_log
+
+if ($select) {
+--eval $select INTO OUTFILE '$MYSQLTEST_VARDIR/tmp/after_explain.txt'
+--diff_files '$MYSQLTEST_VARDIR/tmp/before_explain.txt' '$MYSQLTEST_VARDIR/tmp/after_explain.txt'
+--remove_file '$MYSQLTEST_VARDIR/tmp/before_explain.txt'
+--remove_file '$MYSQLTEST_VARDIR/tmp/after_explain.txt'
+}
+
+FLUSH STATUS;
+FLUSH TABLES;
+if ($select) {
+--disable_result_log
+--eval $select
+--enable_result_log
+--echo # Status of "equivalent" SELECT query execution:
+SHOW STATUS WHERE (Variable_name LIKE 'Sort%' OR
+                   Variable_name LIKE 'Handler_read_%' OR
+                   Variable_name = 'Handler_write' OR
+                   Variable_name = 'Handler_update' OR
+                   Variable_name = 'Handler_delete') AND Value <> 0;
+}
+
+FLUSH STATUS;
+FLUSH TABLES;
+--eval $query
+--echo # Status of testing query execution:
+SHOW STATUS WHERE (Variable_name LIKE 'Sort%' OR
+                   Variable_name LIKE 'Handler_read_%' OR
+                   Variable_name = 'Handler_write' OR
+                   Variable_name = 'Handler_update' OR
+                   Variable_name = 'Handler_delete') AND Value <> 0;
+
+--let $query=
+--let $select=
+--let $no_rows=
+
+--enable_query_log
+
+--echo

mysql-test/t/myisam_explain_non_select_all.test

+# 
+# Run explain_non_select.inc on MyISAM with all of the so-called 6.0 features.
+#
+
+#--source include/have_semijoin.inc
+#--source include/have_materialization.inc
+#--source include/have_firstmatch.inc
+#--source include/have_loosescan.inc
+#--source include/have_index_condition_pushdown.inc
+#--source include/have_mrr.inc
+
+#set optimizer_switch='semijoin=on,materialization=on,firstmatch=on,loosescan=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=off';
+
+set @save_storage_engine= @@session.default_storage_engine;
+set session default_storage_engine = MyISAM;
+--let $json = 0
+--let $validation = 0
+--source include/explain_non_select.inc
+set default_storage_engine= @save_storage_engine;
+
+set optimizer_switch=default;