ClickHouse DB::Exception: No such projection in table

The "DB::Exception: No such projection in table" error in ClickHouse is raised when you reference a projection by name that does not exist on the specified table. This typically happens during operations like ALTER TABLE ... MATERIALIZE PROJECTION, ALTER TABLE ... DROP PROJECTION, or ALTER TABLE ... CLEAR PROJECTION. The error code is NO_SUCH_PROJECTION_IN_TABLE.

Impact

The ALTER statement fails, and no changes are made to the table. This is a metadata-level error that does not affect the table's data or the operation of any existing projections.

Common Causes

1. Typo in the projection name -- misspelling the projection name in ALTER statements.
2. Projection was already dropped -- attempting to drop or materialize a projection that was previously removed.
3. Projection was never created -- referencing a projection that was planned but not yet added to the table.
4. Wrong table -- the projection exists on a different table than the one referenced in the statement.
5. Case sensitivity -- projection names are case-sensitive in ClickHouse. proj_A and proj_a are different names.

Troubleshooting and Resolution Steps

1. List all projections on the table:

   SELECT name, type_full, sorting_key
   FROM system.projection_parts
   WHERE database = 'default' AND table = 'your_table'
   GROUP BY name, type_full, sorting_key;
   

2. Check the table definition for projection declarations:

   SHOW CREATE TABLE your_table;
   

   The output will include all projection definitions.

3. Verify the exact projection name (case-sensitive):

   -- If the projection is named 'proj_by_date', this will fail:
   ALTER TABLE your_table MATERIALIZE PROJECTION Proj_By_Date;
   
   -- Use the exact name:
   ALTER TABLE your_table MATERIALIZE PROJECTION proj_by_date;
   

4. Use IF EXISTS to make the operation idempotent:

   ALTER TABLE your_table DROP PROJECTION IF EXISTS proj_name;
   

5. If the projection needs to be created first, add it:

   ALTER TABLE your_table ADD PROJECTION proj_name (
       SELECT * ORDER BY some_column
   );
   -- Then materialize it for existing data
   ALTER TABLE your_table MATERIALIZE PROJECTION proj_name;
   

Best Practices

• Use IF EXISTS in DROP PROJECTION statements, especially in migration scripts, to handle idempotency.
• Maintain documentation of projections defined on each table, including their exact names.
• After adding a projection, remember to MATERIALIZE PROJECTION to apply it to existing data parts -- otherwise it only applies to newly inserted data.
• Use SHOW CREATE TABLE as the source of truth for projection names rather than relying on memory.

Frequently Asked Questions

Q: Does MATERIALIZE PROJECTION create the projection?
A: No. MATERIALIZE PROJECTION builds projection data for existing parts. The projection must first be defined with ALTER TABLE ... ADD PROJECTION or in the original CREATE TABLE statement. MATERIALIZE only processes parts that do not yet have the projection materialized.

Q: Are projection names case-sensitive?
A: Yes. Projection names in ClickHouse are case-sensitive. my_proj and My_Proj are treated as different names.

Q: Can I rename a projection?
A: ClickHouse does not support renaming projections directly. You would need to drop the existing projection and add a new one with the desired name.

Q: What happens to data if I drop a projection?
A: Dropping a projection only removes the pre-computed projection data. The base table data is completely unaffected. The projection data is cleaned up during subsequent merges.