Allow EXPLAIN in read-only mode

Using `EXPLAIN` when `transaction_read_only` is set to `on` fails with
an error. This is fixed by explicitly setting the `check_read_only`
flag.

src/process_utility.c

@@ -4859,6 +4859,7 @@ process_ddl_command_start(ProcessUtilityArgs *args)
 			handler = preprocess_execute;
 			break;
 		case T_ExplainStmt:
+			check_read_only = false;
 			handler = process_explain_start;
 			break;
 

tsl/test/expected/read_only.out

@@ -8,7 +8,26 @@
 -- create_hypertable()
 --
 CREATE TABLE test_table(time bigint NOT NULL, device int);
+-- EXPLAIN should work in read-only mode, when enabling in transaction.
+START TRANSACTION;
+SET transaction_read_only TO on;
+EXPLAIN (COSTS OFF) SELECT * FROM test_table;
+       QUERY PLAN       
+------------------------
+ Seq Scan on test_table
+(1 row)
+
+ROLLBACK;
 SET default_transaction_read_only TO on;
+-- EXPLAIN should work in read-only mode, even when using the default.
+START TRANSACTION;
+EXPLAIN (COSTS OFF) SELECT * FROM test_table;
+       QUERY PLAN       
+------------------------
+ Seq Scan on test_table
+(1 row)
+
+ROLLBACK;
 \set ON_ERROR_STOP 0
 SELECT * FROM create_hypertable('test_table', 'time');
 ERROR:  cannot execute create_hypertable() in a read-only transaction

tsl/test/sql/read_only.sql

@@ -12,8 +12,19 @@
 --
 CREATE TABLE test_table(time bigint NOT NULL, device int);
 
+-- EXPLAIN should work in read-only mode, when enabling in transaction.
+START TRANSACTION;
+SET transaction_read_only TO on;
+EXPLAIN (COSTS OFF) SELECT * FROM test_table;
+ROLLBACK;
+
 SET default_transaction_read_only TO on;
 
+-- EXPLAIN should work in read-only mode, even when using the default.
+START TRANSACTION;
+EXPLAIN (COSTS OFF) SELECT * FROM test_table;
+ROLLBACK;
+
 \set ON_ERROR_STOP 0
 SELECT * FROM create_hypertable('test_table', 'time');
 \set ON_ERROR_STOP 1
@@ -213,3 +224,4 @@ SELECT remove_retention_policy('test_table');
 SELECT add_job('now','12h');
 SELECT alter_job(1,scheduled:=false);
 SELECT delete_job(1);
+