= Clickhouse read-only mode =

You need to be careful when you use a read-only Clickhouse user in your APIs. The default read-only mode (readonly = 1) will effectively prevent you from changing any query settings, e.g. join_use_nulls, enable_unaligned_array_join, etc. In order to make this work with the read-only mode, you need to set up the readonly flag to 2 in the users.xml:

<profiles>
  <readonly>
    <readonly>2</readonly>
  </readonly>
</profiles>

<users>
  <apiuser>
    <profile>readonly</profile>
  </ltvapi>
</users>

With the above in place, your queries will be able to setup query settings yet again.

From the official documentation:

Read-only mode is enabled by the readonly setting (not to confuse with readonly constraint type):

• readonly=0: No read-only restrictions.
• readonly=1: Only read queries are allowed and settings cannot be changed unless changeable_in_readonly is set.
• readonly=2: Only read queries are allowed, but settings can be changed, except for readonly setting itself.