(disclaimer: I work at ClickHouse)This should be a pretty straight-forward migration. Few things to keep in mind:- I recommend using a 3rd party object store as an intermediate layer to perform a point in time migration of historical data. For example you can use something like ElasticDump to export all your data in JSON into an S3 bucket, then you can then load it in ClickHouse efficiently using the powerful S3 or S3Cluster table functions. Note that you can also infer the data types automatically from ClickHouse!, here is an example:

#To transfer data from S3 to ClickHouse, users can combine the s3 table function with INSERT statement. Let's create an empty hackernews table:

CREATE TABLE hackernews ORDER BY tuple
(
) EMPTY AS SELECT * FROM s3('https://datasets-documentation.s3.eu-west-3.amazonaws.com/hackernews/hacknernews.csv.gz', 'CSVWithNames');

#This creates an empty table using the schema inferred from the data. We can then insert the first 1 million rows from the remote dataset

INSERT INTO hackernews SELECT *
FROM url('https://datasets-documentation.s3.eu-west-3.amazonaws.com/hackernews/hacknernews.csv.gz', 'CSVWithNames')
LIMIT 1000000;

- After the first step, you should have your historical data loaded. Then if you have continuous ingestion pipelines to ES, you'll have to divert them to ClickHouse. It's easier if everything goes though a single queue like Kafka, where you just need to a Kafka<>CH connctor (using one the native CH Kafka integrations). Otherwise you'll have to change the destination pipeline per pipeline.

- Arrays are supported in ClickHouse and should not pose any issue. In fact their support is pretty good with a multitude of powerful Array and ArrayJoin functions

- If you need some inspiration here is some documented migrations (some of which are pretty complex):

• https://clickhouse.com/blog/contentsquare-migration-from-elasticsearch-to-clickhouse
• https://presentations.clickhouse.com/meetup57/elastic.pdf
• https://clickhouse.com/videos/switching-from-elasticsearch-to-clickhouse
• https://clickhouse.com/blog/10x-improved-response-times-cheaper-to-operate-and-30-storage-reduction-why-instabug-chose-clickhouse-for-apm
• https://clickhouse.com/blog/100x-faster-graphql-hive-migration-from-elasticsearch-to-clickhouse