MDEV-13359 Enable ALTER TABLE...ALGORITHM=INPLACE for compressed columns

innobase_rec_reset(): Remove. This function was introduced in the
InnoDB Plugin for MySQL 5.1, which later evolved into MySQL 5.5.
There used to be a bug that ADD UNIQUE INDEX would not always correctly
report the duplicate key value of the secondary index. This function
ensured that instead of reporting total garbage values, InnoDB
would report NULL.

It looks like the function was made unnecessary in MySQL 5.6.6 by
mysql/mysql-server@d143097

The corresponding test was subsequently adjusted in
mysql/mysql-server@fde80cf

The ALTER TABLE tests were imported to MariaDB as part of MDEV-13625,
and these tests do pass with this change.

The unnecessary function did not do any harm before MDEV-11371 introduced
compressed columns.

One question remains: What if we needed to report a duplicate key value
for a compressed column? The simple answer is that the test
main.column_compression demonstrates that no indexes can be defined
on compressed columns.

storage/innobase/handler/handler0alter.cc

@@ -727,13 +727,6 @@ ha_innobase::check_if_supported_inplace_alter(
 
 			DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
 		}
-
-		/* Disable online ALTER TABLE for compressed columns until
-		MDEV-13359 - "Online ALTER TABLE will be disabled for compressed columns"
-		is fixed. */
-		if (field->compression_method()) {
-			DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
-		}
 	}
 
 	ulint n_indexes = UT_LIST_GET_LEN((m_prebuilt->table)->indexes);
@@ -1032,12 +1025,6 @@ ha_innobase::check_if_supported_inplace_alter(
 			    || (ha_alter_info->handler_flags
 				& Alter_inplace_info::ADD_COLUMN));
 
-		/* Disable online ALTER TABLE for compressed columns until
-		MDEV-13359 - "Online ALTER TABLE will be disabled for compressed columns"
-		is fixed. */
-		if (cf->compression_method()) {
-			DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
-		}
 		if (const Field* f = cf->field) {
 			/* This could be changing an existing column
 			from NULL to NOT NULL. */
@@ -1982,21 +1969,6 @@ innobase_row_to_mysql(
 	}
 }
 
-/*************************************************************//**
-Resets table->record[0]. */
-void
-innobase_rec_reset(
-/*===============*/
-	TABLE*			table)		/*!< in/out: MySQL table */
-{
-	uint	n_fields	= table->s->fields;
-	uint	i;
-
-	for (i = 0; i < n_fields; i++) {
-		table->field[i]->set_default();
-	}
-}
-
 /*******************************************************************//**
 This function checks that index keys are sensible.
 @return 0 or error number */

storage/innobase/include/handler0alter.h

@@ -53,14 +53,6 @@ innobase_row_to_mysql(
 	const dtuple_t*		row)	/*!< in: InnoDB row */
 	MY_ATTRIBUTE((nonnull));
 
-/*************************************************************//**
-Resets table->record[0]. */
-void
-innobase_rec_reset(
-/*===============*/
-	struct TABLE*		table)		/*!< in/out: MySQL table */
-	MY_ATTRIBUTE((nonnull));
-
 /** Generate the next autoinc based on a snapshot of the session
 auto_increment_increment and auto_increment_offset variables. */
 struct ib_sequence_t {

storage/innobase/row/row0merge.cc

@@ -4867,12 +4867,6 @@ row_merge_build_indexes(
 		}
 	}
 
-	/* Reset the MySQL row buffer that is used when reporting
-	duplicate keys.
-
-	This is likely reason for a problem described in MDEV-13359. */
-	innobase_rec_reset(table);
-
 	if (global_system_variables.log_warnings > 2) {
 		sql_print_information("InnoDB: Online DDL : Start reading"
 				      " clustered index of the table"