DROP TABLE IF EXISTS array_codec_test SYNC

create table array_codec_test( number UInt64, arr Array(UInt64) ) Engine=MergeTree ORDER BY number;
INSERT INTO array_codec_test SELECT number,  arrayMap(i -> number + i, range(100)) from numbers(10000000);


/****  Default LZ4  *****/

OPTIMIZE TABLE array_codec_test FINAL;
--- Elapsed: 3.386 sec.


SELECT * FROM system.columns WHERE (table = 'array_codec_test') AND (name = 'arr')
/*
Row 1:
──────
database:                default
table:                   array_codec_test
name:                    arr
type:                    Array(UInt64)
position:                2
default_kind:         
default_expression:   
data_compressed_bytes:   173866750
data_uncompressed_bytes: 8080000000
marks_bytes:             58656
comment:              
is_in_partition_key:     0
is_in_sorting_key:       0
is_in_primary_key:       0
is_in_sampling_key:      0
compression_codec:    
*/



/****** Delta, LZ4 ******/

ALTER TABLE array_codec_test MODIFY COLUMN arr Array(UInt64) CODEC (Delta, LZ4);

OPTIMIZE TABLE array_codec_test FINAL
--0 rows in set. Elapsed: 4.577 sec.

SELECT * FROM system.columns WHERE (table = 'array_codec_test') AND (name = 'arr')

/*
Row 1:
──────
database:                default
table:                   array_codec_test
name:                    arr
type:                    Array(UInt64)
position:                2
default_kind:         
default_expression:   
data_compressed_bytes:   32458310
data_uncompressed_bytes: 8080000000
marks_bytes:             58656
comment:              
is_in_partition_key:     0
is_in_sorting_key:       0
is_in_primary_key:       0
is_in_sampling_key:      0
compression_codec:       CODEC(Delta(8), LZ4)
*/

create table test_codec_speed engine=MergeTree
ORDER BY tuple()
as select cast(now() + rand()%2000 + number, 'DateTime') as x from numbers(1000000000);

option 1: CODEC(LZ4) (same as default)
option 2: CODEC(DoubleDelta) (`alter table test_codec_speed modify column x DateTime CODEC(DoubleDelta)`);
option 3: CODEC(T64, LZ4) (`alter table test_codec_speed modify column x DateTime CODEC(T64, LZ4)`)
option 4: CODEC(Delta, LZ4) (`alter table test_codec_speed modify column x DateTime CODEC(Delta, LZ4)`)
option 5: CODEC(ZSTD(1)) (`alter table test_codec_speed modify column x DateTime CODEC(ZSTD(1))`)
option 6: CODEC(T64, ZSTD(1)) (`alter table test_codec_speed modify column x DateTime CODEC(T64, ZSTD(1))`)
option 7: CODEC(Delta, ZSTD(1)) (`alter table test_codec_speed modify column x DateTime CODEC(Delta, ZSTD(1))`)
option 8: CODEC(T64, LZ4HC(1)) (`alter table test_codec_speed modify column x DateTime CODEC(T64, LZ4HC(1))`)
option 9: CODEC(Gorilla) (`alter table test_codec_speed modify column x DateTime CODEC(Gorilla)`)

Result may be not 100% reliable (checked on my laptop, need to be repeated in lab environment)


OPTIMIZE TABLE test_codec_speed FINAL (second run - i.e. read + write the same data)
1) 17 sec.
2) 30 sec.
3) 16 sec
4) 17 sec
5) 29 sec
6) 24 sec
7) 31 sec
8) 35 sec
9) 19 sec

compressed size
1) 3181376881
2) 2333793699
3) 1862660307
4) 3408502757
5) 2393078266
6) 1765556173
7) 2176080497
8) 1810471247
9) 2109640716

select max(x) from test_codec_speed
1) 0.597
2) 2.756 :(
3) 1.168
4) 0.752
5) 1.362
6) 1.364
7) 1.752
8) 1.270
9) 1.607

CREATE TABLE test_table AS source_table ENGINE=MergeTree() PARTITION BY ...;

ALTER TABLE test_table ATTACH PARTITION ID '20210120' FROM source_table;

ALTER TABLE test_table MODIFY COLUMN column_a CODEC(ZSTD(2));
ALTER TABLE test_table ADD COLUMN column_new UInt32
                         DEFAULT toUInt32OrZero(column_old) CODEC(T64,LZ4);

ALTER TABLE test_table UPDATE column_a=column_a, column_new=column_new WHERE 1;

SELECT * FROM system.mutations;

KILL MUTATION WHERE ...

SELECT
    database,
    table,
    count() AS parts,
    uniqExact(partition_id) AS partition_cnt,
    sum(rows),
    formatReadableSize(sum(data_compressed_bytes) AS comp_bytes) AS comp,
    formatReadableSize(sum(data_uncompressed_bytes) AS uncomp_bytes) AS uncomp,
    uncomp_bytes / comp_bytes AS ratio
FROM system.parts
WHERE active
GROUP BY
    database,
    table
ORDER BY comp_bytes DESC

SELECT
  database,
  table,
  column,
  type,
  sum(rows) AS rows,
  sum(column_data_compressed_bytes) AS compressed_bytes,
  formatReadableSize(compressed_bytes) AS compressed,
  formatReadableSize(sum(column_data_uncompressed_bytes)) AS uncompressed,
  sum(column_data_uncompressed_bytes) / compressed_bytes AS ratio,
  any(compression_codec) AS codec
FROM system.parts_columns AS pc
LEFT JOIN system.columns AS c
ON (pc.database = c.database) AND (c.table = pc.table) AND (c.name = pc.column)
WHERE (database LIKE '%') AND (table LIKE '%') AND active
GROUP BY
  database,
  table,
  column,
  type
ORDER BY database, table, sum(column_data_compressed_bytes) DESC