FAQ

Q: Why does CHYT have cliques, while regular ClickHouse has nothing analogous? What is a clique anyway?

A: There is a dedicated Concepts article about this.

Q: I get one of the DB::NetException: Connection refused, DB::Exception: Attempt to read after eof: while receiving packet from errors. What does it mean?

A: This usually means that an instance has left for one reason or another. You can view the counters of the number of aborted/failed jobs in the operation UI. If there are (recent) jobs aborted due to preemption, it means that the clique does not have enough resources. If there are (recent) failed jobs, contact the administrator.

Q: What does the Subquery exceeds data weight limit: XXX > YYY error mean?

A: See the max_data_weight_per_subquery option in the article about configuration.

Q: How do I save data to a table?

A: There are INSERT INTO and CREATE TABLE functions. Learn more about them in the Working with YTsaurus tables section.

Q: How do I load geo-dicts in my own clique?

A: When starting any clique, you can specify the --cypress-geodata-path option that enables you to specify the path to geo-dicts in Cypress. For more information about this option, see the How to try article.

Q: Can CHYT process dates in TzDatetime format?

A: CHYT can process dates in TzDatetime format just as much as regular ClickHouse (all the same functions are available). You will have to store the data as strings or numbers and convert when reading and writing. Date extraction example:

toDate(reinterpretAsInt64(reverse(unhex(substring(hex(payment_dt), 1, 8)))))

Q: How do I move a table to an SSD?

A: First, make sure that your YTsaurus account has a quota for the ssd_blobs medium. To do this, go to the Accounts page, switch your medium type to ssd_blobs, and enter your account name. If there is no quota in the ssd_blobs medium, request it from the administrator.

After obtaining the quota for the ssd_blobs medium, you will need to change the value of the primary_medium attribute, and the data will be moved to the corresponding medium in the background.

For static tables, you can force a move using the Merge operation.

yt set //home/dev/test_table/@primary_medium ssd_blobs
yt merge --mode auto --spec '{"force_transform"=true;}' --src //home/dev/test_table --dst //home/dev/test_table

If the table is dynamic, to change the medium, you must first unmount the table,
set the attribute, and then mount it back:

yt unmount-table //home/dev/test_table --sync
yt set //home/dev/test_table/@primary_medium ssd_blobs
yt mount-table //home/dev/test_table --sync

You can additionally speed up moving using forced_compaction, but using this method creates a heavy load on the cluster and is strongly not recommended.

To check that the table really changed the medium, you can use the command:

yt get //home/dev/test_table/@resource_usage

{
    "tablet_count" = 0;
    "disk_space_per_medium" = {
        "ssd_blobs" = 930;
    };
    "tablet_static_memory" = 0;
    "disk_space" = 930;
    "node_count" = 1;
    "chunk_count" = 1;
}

Q: Is the ClickHouse SAMPLE clause supported?

A: CHYT supports the SAMPLE clause. The difference is that CHYT ignores the OFFSET ... command, so you cannot get a sample from another part of the selected data.

Example:

SELECT count(*) FROM "//tmp/sample_table" SAMPLE 0.05;

SELECT count(*) FROM "//tmp/sample_table" SAMPLE 1/20;

SELECT count(*) FROM "//tmp/sample_table" SAMPLE 100500;

Q: How do I get the table name in a query?

A: You can use the $table_name and $table_path virtual columns. For more information about the virtual columns, see Working with YTsaurus tables.