MDEV-23805 Make Online DDL to Instant DDL when table is emptybb-10.6-MDEV-23805

- In ha_innobase::prepare_inplace_alter_table(), InnoDB should
check whether the table is empty. If the table is empty then
server should avoid downgrading the MDL after prepare phase.
It is more like instant alter, does change only in dicationary
and metadata.

- Changed few debug test case to make non-empty DDL table

1 files changed, 6 insertions, 5 deletions

diff --git a/mysql-test/main/innodb_mysql_sync.test b/mysql-test/main/innodb_mysql_sync.test
index 4026080c4b4..56e4da34031 100644
--- a/mysql-test/main/innodb_mysql_sync.test
+++ b/mysql-test/main/innodb_mysql_sync.test
@@ -176,6 +176,7 @@ DROP DATABASE db1;
 --echo # Test 2: Primary index (implicit), should block writes.
 
 CREATE TABLE t1(a INT NOT NULL, b INT NOT NULL) engine=innodb;
+INSERT INTO t1 VALUES(1, 1);
 SET DEBUG_SYNC= "alter_table_inplace_after_lock_downgrade SIGNAL manage WAIT_FOR query";
 --echo # Sending:
 --send ALTER TABLE t1 ADD UNIQUE INDEX(a), LOCK=SHARED
@@ -185,13 +186,13 @@ SET DEBUG_SYNC= "now WAIT_FOR manage";
 USE test;
 SELECT * FROM t1;
 --echo # Sending:
---send UPDATE t1 SET a=NULL
+--send UPDATE t1 SET a=2
 
 connection con2;
 --echo # Waiting for SELECT to be blocked by the metadata lock on t1
 let $wait_condition= SELECT COUNT(*)= 1 FROM information_schema.processlist
   WHERE state= 'Waiting for table metadata lock'
-  AND info='UPDATE t1 SET a=NULL';
+  AND info='UPDATE t1 SET a=2';
 --source include/wait_condition.inc
 SET DEBUG_SYNC= "now SIGNAL query";
 
@@ -215,13 +216,13 @@ connection con1;
 SET DEBUG_SYNC= "now WAIT_FOR manage";
 SELECT * FROM t1;
 --echo # Sending:
---send UPDATE t1 SET a=NULL
+--send UPDATE t1 SET a=3
 
 connection con2;
 --echo # Waiting for SELECT to be blocked by the metadata lock on t1
 let $wait_condition= SELECT COUNT(*)= 1 FROM information_schema.processlist
   WHERE state= 'Waiting for table metadata lock'
-  AND info='UPDATE t1 SET a=NULL';
+  AND info='UPDATE t1 SET a=3';
 --source include/wait_condition.inc
 SET DEBUG_SYNC= "now SIGNAL query";
 
@@ -230,7 +231,7 @@ connection default;
 --reap
 
 connection con1;
---echo # Reaping: UPDATE t1 SET a=NULL
+--echo # Reaping: UPDATE t1 SET a=3
 --reap
 
 --echo # Test 4: Secondary unique index, should not block reads.