Big Column Compression

TenDB provides compressed attributes for BLOB related columns. It is beneficial to reduce storage cost and improve main memory utilization, thereby increasing IO efficiency. This article introduces TenDB's new features in column compression.

Create Compressed Column

We can specify compressed attribute to some columns when creating table (only applies when the storage engine is InnoDB). Columns with compressed attribute will compress data when it is inserted, and decompress data when querying. The compression/decompression operation uses the ZLIB algorithm and is transperant to developers.

Here is an example to create compressd columns (C2 and C3):

create table t1 (
  C1 int,
  C2 blob compressed,
  C3 text character set gbk compressed,
  C4 blob

The compressed attribute is limited to columns of the following data types: TINYBLOB, BLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT and JSON

Please note that, we can not add compressed attribute to VARCHAR and VARBINARY. Instead, we can use alter table to convert VARCHAR/VARBINARY columns to BLOB/TEXT COMPRESSED ones if we want to use compressed columns. The example is shown below:

create table t1 (
  id int,
  vc varchar(1000)

insert into t1(vc) values(repeat('abc',200)),(repeat('xyz',300));

alter table t1 modify vc text compressed;

When this is done, the original varchar data in t1 is compressed and written to the text compressed column.

You can also modify a BLOB column to a BLOB COMPRESSED one. Similarly, the data would be inserted into the column after compression.

Index on Compressed Columns

Columns with compressed attribute can not be indexed directly. However, it can be indexed on its generated columns. For instance, you can index on a virtual generated prefix column based on a BLOB column:

  vcol VARCHAR(20) GENERATED ALWAYS AS (left(bcol, 10)),
  index idx(scol)

Or you can index on a generated column based on one JSON element:

    vcol VARCHAR(20) GENERATED ALWAYS AS (jcol->"$.name"),
    index idx(scol)

How to Know a Column is Compressed

We can use show create table to get whether a column is compressed or not. The compressed column is shown with /*!99104 COMPRESSED */:

mysql> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `bcol` blob /*!99104 COMPRESSED */

Or use describe table to see if there exists COMPRESSED COLUMN in the Extra field.

mysql> describe t1\G
*************************** 1. row ***************************
  Field: bcol
   Type: blob
   Null: YES
Default: NULL

Compression Threshold Control

TenDB provides InnoDB global variable innodb_min_blob_compress_length. InnoDB only do compression when the original length of data exceeded this value. This variable is 256 by default.

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