如何在Clickhouse数据库的同一选择查询中查询多个数据库？

Question

I have a clickhouse database. It contains multiple databases. The tables inside the databases are identical.

For example DB1 has the table "Table1", DB2 also has the table "Table1" (Here the databases are different, Tables are also different but they have identical schema and contain similar type of information).

Is there a way that i can write a query to get the information from all the different tables from all the different databases optimally? Currently i am querying each table individually and doing an Union among them.

Answer 1

You could set up a special table with Merge engine (not to be confused with MergeTree). Querying it will do the the same UNION ALL transparently. It does not support spanning through multiple databases though, so you still might end up having multiple of those or moving the tables into one db.

Answer 2

I would propose to create one more DB:

create database <your_db_name> on cluster <your_cluster_name>;

Then create a Merge table for each of you existing DB:

CREATE TABLE <your_db_name>.<merge_table_name> On cluster <your_cluster_name> (<fields list you need to deal with>)  ENGINE=Merge(<existing_db_name>, '<regex>');

Create a View for each of the Merge tables (using a view you can execute any aggregate functions, group by and etc.):

CREATE VIEW <your_db_name>.view_<merge_table_name> on cluster <your_cluster_name> (<list of fields>) AS SELECT <list of fields> FROM <your_db_name>.<merge_table_name>;

Finally create a "result" Merge table that combines all the views newly created:

CREATE TABLE <your_db_name>.<result_table_name> on cluster <your_cluster_name> (<list of fields>)  ENGINE=Merge(<your_db_name>, '<regex>');

Name the tables and views in such a way that your regular expression matches all you need and make sure you avoid loops.