ClickHouse DB::Exception: Too many temporary columns

The "DB::Exception: Too many temporary columns" error in ClickHouse occurs when the number of temporary columns generated during query execution exceeds the max_temporary_columns setting. The TOO_MANY_TEMPORARY_COLUMNS error code indicates that ClickHouse's internal processing has created more intermediate columns than allowed, typically as a result of complex expressions, numerous computed fields, or wide result transformations.

Impact

The query is terminated during execution when the temporary column count exceeds the threshold. This affects queries with many computed expressions, complex transformations, or those that manipulate wide intermediate result sets. The error prevents excessive memory usage from the overhead of maintaining too many intermediate columns in memory simultaneously.

Common Causes

1. Queries with a large number of computed expressions or aliases in the SELECT clause
2. Complex expressions that decompose into many intermediate columns during evaluation
3. Queries involving wide JOINs that combine many columns from multiple tables
4. Use of functions that internally expand into multiple temporary columns (e.g., certain array or tuple operations)
5. The max_temporary_columns setting is configured too low for the query's complexity

Troubleshooting and Resolution Steps

1. Check the current temporary column limit:

   SELECT name, value FROM system.settings
   WHERE name IN ('max_temporary_columns', 'max_temporary_non_const_columns');
   

2. Increase the limit if the query is legitimate:

   SET max_temporary_columns = 5000;
   SELECT ...;
   

3. Reduce the number of computed columns in a single query. Split the work into multiple steps:

   -- Instead of one query with 200 computed columns:
   CREATE TEMPORARY TABLE step1 AS
   SELECT id, computed_col1, computed_col2, ..., computed_col50 FROM my_table;
   
   CREATE TEMPORARY TABLE step2 AS
   SELECT id, computed_col51, computed_col52, ..., computed_col100 FROM my_table;
   
   SELECT s1.*, s2.* FROM step1 s1 JOIN step2 s2 USING (id);
   

4. Simplify expressions that may generate excessive intermediate columns:

   -- Complex nested expressions may generate many temporaries
   -- Consider pre-computing values or using simpler alternatives
   

5. Select fewer columns from wide joins. Only include columns you actually need in the final result:

   -- Instead of:
   SELECT * FROM wide_table1 JOIN wide_table2 USING (key);
   
   -- Select specifically:
   SELECT t1.col1, t1.col2, t2.col3 FROM wide_table1 t1 JOIN wide_table2 t2 USING (key);
   

6. Check max_temporary_non_const_columns as well, which limits only non-constant temporary columns and may be the actual constraint:

   SET max_temporary_non_const_columns = 5000;
   

Best Practices

• Keep the number of expressions in a single query manageable by splitting complex transformations into stages.
• Avoid SELECT * when joining multiple wide tables.
• Use max_temporary_columns as a guardrail in user profiles to catch overly complex queries early.
• Design queries to compute only what is needed rather than generating large intermediate result sets.
• When working with wide tables, consider vertical table design patterns that keep individual tables narrower.

Frequently Asked Questions

Q: What is the default value of max_temporary_columns?
A: The default is 0 (unlimited). If you encounter this error, the limit has been set in your server configuration or user profile.

Q: What is the difference between max_temporary_columns and max_temporary_non_const_columns?
A: max_temporary_columns limits the total number of temporary columns including constants, while max_temporary_non_const_columns limits only non-constant temporary columns. Constant columns (e.g., literal values) use minimal memory, so the non-const variant is often the more meaningful constraint.

Q: How do I know how many temporary columns my query generates?
A: There is no direct way to query this before execution. However, the error message will report the count when the limit is exceeded. You can also use EXPLAIN PIPELINE to get a sense of query complexity.

Q: Are temporary columns the same as the columns in my table?
A: No. Temporary columns are internal intermediate columns created during query processing for computed expressions, function evaluations, and other transformations. They exist only during query execution and are not persisted.