How can data be transferred when migrating a materialized view from Greenplum to ClickHouse?

To transfer data from a materialized view in Greenplum to ClickHouse, several approaches can be used:

1. Export and import via CSV or Parquet:

   • Export data from Greenplum to a CSV or Parquet file using COPY or external utilities.
   • Then load these files into ClickHouse using clickhouse-client or through the HTTP interface.

2. Using intermediate ETL tools:

   • For example, Apache NiFi, Airflow, or specialized connectors that can read from Greenplum and write to ClickHouse.

3. Direct connection and streaming:

   • If possible, write a script in Python or another language that reads data from Greenplum (via psycopg2) and inserts it into ClickHouse (via clickhouse-driver).

4. Using external tables or FDW (Foreign Data Wrapper):

   • Greenplum can set up external tables, but such integration is usually not available for ClickHouse, so export/import is more common.

Example of exporting from Greenplum to CSV:

COPY materialized_view_name TO '/path/to/export.csv' WITH CSV HEADER;

Then loading into ClickHouse:

clickhouse-client --query="INSERT INTO target_table FORMAT CSV" < /path/to/export.csv