## Scope

If you heard about Clickhouse and you are wondering how to test with your residing data in Redshift, here is a command that will show you a few tips to make you speed up.

Update (July 4th): There is a serie of posts about Clickhouse vs Redshift comparisons, the first post is this one.

The standard wat to move your data out of Redshift is by using UNLOAD command, which pushes the output into S3 files. Not surprisingly, Redshift does not support COPY (<query>) TO STDOUT, which could make life easier (as it Postgres version 8.0.2 based, quite ol’). Info about this, here.

Clickhouse supports several engines but so far, you will for sure start with MergeTree. The supported types are more finite, although they should be enough for plain analytics. It’s recommended to add sampling support at table creation, in the engine parameters through corresponding hash function with the column type that return unsigned integers after the key definition. In this case I’ve choosen cityHash64 as it is not cryptographic, it has a decent accuracy and better performance.

The table in CH is the following:

CREATE TABLE thenewdb.thetable (
normdate Date,
id String,
datefield DateTime,
(... many others ...)
data String
)
ENGINE = MergeTree(normdate,cityHash64(id), (datefield, id,cityHash64(id)),8192);


NOTE: The engine parameters are: a date column, the optional sampling expression (cityHash64) the primary key (datefield,id) and the index granularity.

The table in Redshift is:

     Column     |            Type             | Modifiers
----------------+-----------------------------+-----------
id             | character varying(32)       | not null
datefield      | timestamp without time zone | not null
(... other columns...)
data           | character varying(8192)     |
Indexes:
"thetable_pkey1" PRIMARY KEY, btree (id)


ClickHouse requires a Date column, which ends up to be an additional column in your table structure. For more information, check out the MergeTree doc.

## The magic

• Open a screen/ tmux session.

• Execute the command:

time psql -h rs1.garbagestring.redshift.amazonaws.com \
-p 5439 -U redshift thedatabase \
-Antq --variable="FETCH_COUNT=1000000" -F $'\t' <<EOF | \ clickhouse-client --database thenewdb --query="INSERT INTO thenewdb.thetable FORMAT TabSeparated" select trunc(datefield), id, datefield::timestamp(0) , store_id , (... many columns more ... ) regexp_replace(data,'\\t|\\n','') from theoriginaltable EOF  ## Calculation of RAM MergeTree engine is indeed an interesting implementation. It is not an LSM as it does not process in memtables nor neither log. It process the data in batches and write directly to the file system, consuming a significant amount of RAM at the cost of saving disk operations (and occasionally CPU cycles) by background workers that do the merges. A common error when you run out of memory due to this merge processes eating RAM is: Code: 240. DB::Exception: Allocator: Cannot mremap., errno: 12, strerror: Cannot allocate memory  The reason on why this happens is due to the RAM consumed on background merges. There are five elements to have in mind in order to calculate the needed memory: • background_pool_size is 6, determining the maximum number of background merges. • Maximum number of merge pieces during merge (default 100) • block size for the merger (8192 rows) • average size of row uncompressed • maximum overhead memory allocation for buffers (2) You can assume a row size of 1024 bytes and multiply all of the above together. i.e. SELECT formatReadableSize( 2* 6 * 100 * 8192 * 1024); The current issue is that the merge algorithm process by row instead each column separately, and is expected to have a performance gain. You can try the vertical algorithm by setting enable_vertical_merge_algorithm in the configuration file. So, guessing that you get a row size of 13557 bytes (14k) measured using query 1), you can get an approximate of RAM needed for the block of operations 2). 1) time psql -h rs-clusterandhash.us-east-1.redshift.amazonaws.com\ -p 5439 -U redshift reportdb -Antq --variable="FETCH_COUNT=1000000" -F$'\t' <<EOF | wc -c
select
*
from big_table
LIMIT 1
EOF
13835


2)

SELECT formatReadableSize((((2 * 6) * 100) * 8192) * 13557)
│ 124.12 GiB                                                                         │
└────────────────────────────────────────────────────────────────────────────────────┘


Unfortunately, client can’t handle this properly yet. Even limiting the memory usage with --max_memory_usage 5GB (i.e), you will get a different error like this:

Code: 241. DB::Exception:
DB::Exception: Memory limit (for query) exceeded:
would use 1.00 MiB (attempt to allocate chunk of 1048576 bytes), maximum: 5.00 B.


If the necessary RAM is very close to your current resource, a possible solution would be using ReplacingMergeTree engine, but deduplication is not warranted and indeed you will play in very small limits (you should be very close to the above calculation). Also, there are several settings at engine level for tuning the mergetree engine through configuration at MergeTreeSettings.h

i.e., the bellow will reduce the RAM consumption considerably, at a cost of reducing durability and changing merge algorithm:

    <merge_tree>
<max_suspicious_broken_parts>20</max_suspicious_broken_parts>
<enable_vertical_merge_algorithm>1</enable_vertical_merge_algorithm>
<max_delay_to_insert>5</max_delay_to_insert>
<parts_to_delay_insert>100</parts_to_delay_insert>
</merge_tree>


## The explanation

• Why TabSeparated?

Clickhouse offers several input/output formats, a lot. Even tho, the tab in this case seemed enough for importing plain texts (until a magic JSON with tabs and newlines broke the import).

• Why casting with no microseconds ::timestamp(0)?

CH does not support microseconds.

• Why doing replace regexp_replace(data,'\\t|\\n','')?

We are importing using TSV, which by standard it does not support newlines and obviously, tabs. Unfortunately, is not possible at the moment to use enconding/decoding using base64 for inserting without replacing (by streaming in the data encoded and decode on fly by Clickhouse).

• Why --variable="FETCH_COUNT=1000000"?

This is the sauce. psql will try to place the whole result set in memory, making the box explode within a few minutes after start running. Within this, it creates a server-side cursor, allowing us to import result set bigger than the client machine.

• Why -F $'\t'? Depending on your shell, you may consider this. You need to use a literal tab, which means that it needs to be the character itself. On UNIX Ctrl-V tab should do the thing. You can do a small try abuot this with echo. The option -e enables the interpretation of backslash escapes. Also printf is a clean option for printing special characters. ubuntu@host:~$ echo $'\n' ubuntu@host:~$ echo '\n'
\n
ubuntu@host:~\$ echo -e '\n'



## Rogue numbers

The process itself is consirably fast: it moved a 15GB table into a Clickhouse MergeTree of around 11GB in 20 minutes.

Instance details for RS: dc1.large 15GB RAM, vCPU 2, 2 nodes + 1 coordinator Instance CH: single EC2 r4.2xlarge, volume 3000 iops EBS

I hope you find this tip useful!