OOM when alter column to compression

Got an OOM when doing
ALTER TABLE t1 MODIFY a10 BLOB COLUMN_FORMAT COMPRESSED;

Reproduction:

1,

CREATE TABLE t1 (

a1 BLOB COLUMN_FORMAT COMPRESSED,

a2 BLOB COLUMN_FORMAT COMPRESSED,

a3 BLOB COLUMN_FORMAT COMPRESSED,

a4 BLOB COLUMN_FORMAT COMPRESSED,

a5 BLOB COLUMN_FORMAT COMPRESSED,

a6 BLOB COLUMN_FORMAT COMPRESSED,

a7 BLOB COLUMN_FORMAT COMPRESSED,

a8 BLOB COLUMN_FORMAT COMPRESSED,

a9 BLOB COLUMN_FORMAT COMPRESSED,

a10 BLOB);

2,

DROP PROCEDURE IF EXISTS save_data;

DELIMITER $$

CREATE PROCEDURE save_data()

BEGIN

  DECLARE i INT DEFAULT 1;

  WHILE I<=600000 DO

INSERT INTO t1 values(REPEAT('a', 10000), REPEAT('a', 10000),REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000), REPEAT('a', 10000));

    SET i = i+1;

  END WHILE;

END

$$

delimiter ;

CALL save_data();

(This takes quite a long time)

3,

ALTER TABLE t1 MODIFY a10 BLOB COLUMN_FORMAT COMPRESSED;

I got 30G memory. And this sql eat all of them.

Open

Root cause:

Alter column_format sql goes to copy ddl. For write prebuilt, every copy write will allocate memory from prebuilt->compress_heap in row_compress_column func.

buf = static_cast<byte *>(mem_heap_zalloc(*compress_heap, buf_len));

And this heap is released at the end of the this statement. In my alter case, millions of column is compressed and tons of memory is allocated. It is easy to observe the memory is consumed by mysql server.

Solution: 

@@ -1031,6 +1032,9 @@ static void row_mysql_convert_row_to_innobase(
   ut_ad(prebuilt->template_type == ROW_MYSQL_WHOLE_ROW);
   ut_ad(prebuilt->mysql_template);

+  if (prebuilt->compress_heap)

{ +    row_mysql_prebuilt_free_compress_heap(prebuilt); +  }

It seems free prebuilt->compress_heap before compress a row is a proper way to solve this issue.