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Why can't I see my data in a dictionary in ClickHouse Cloud?

· 2 min read

Dictionaries created in ClickHouse Cloud may experience inconsistency during the initial creation phase. This means that you may not see any data in the dictionary right after creation. However, after several retries, the creation query may land on different replicas, and data will be visible.

This sometimes occurs because the dictionary was created before the part reached the server. As an example:

2024-01-25 13:38:25.615837 - CREATE DICTIONARY received
2024-01-25 13:38:25.626468 - CREATE DICTIONARY finished
2024-01-25 13:38:25.733008 - Part all_0_0_0 downloaded

As you can see, the part only arrived after the dictionary was created. This can be a bigger problem if you are using LIFETIME(MIN 0 MAX 0) because this means that dictionary will never be refreshed automatically. Therefore, the dictionary will remain empty until the command RELOAD DICTIONARIES is executed.

The solution to this issue is to use a SELECT query instead of specifying a source table when creating the dictionary and enabling the setting select_sequential_consistency=1.

Instead of specifying a source table:

table 'test.temp_title_table_1706189903924'
user default password 'PASSWORD'))

Use a SELECT query with select_sequential_consistency=1:

'SELECT songTitle, mappedTitle
FROM test.temp_title_table_1706189903924
SETTINGS select_sequential_consistency=1' USER default PASSWORD ''))

Why does this issue occur?

When you insert data and then create or reload a dictionary, the DDL may reach a replica before the data (or new data) does. This leads to the dictionaries being inconsistent between replicas. Then, depending on which replica receives the query, you may get different results.

Note that the same thing happens when you insert and immediately after read from a table. If you read from a replica that hasn't replicated the data yet, you won't see the newly inserted data. When you need sequential consistency, at the cost of performance (which is why it's generally not recommended to use) you can enable select_sequential_consistency.

The case of dictionaries is a bit trickier since dictionaries don't use the settings from the query, but the settings from the server. As a result, when loading data into the dictionary, even if you SET select_sequential_consistency=1 data may load inconsistently across replicas. Specifying select_sequential_consistency=1 in the dictionary source query allows the dictionary to adhere to this setting even if it's not globally enabled as a server setting.