eval

The eval table function evaluates its argument to a query string, then executes that string as a single SELECT query.

This table function is experimental and disabled by default. Enable it with the allow_experimental_eval_table_function setting:

SET allow_experimental_eval_table_function = 1;

It also requires the analyzer (enable_analyzer, enabled by default).

Syntax

eval(expression)
eval(SELECT ...)

Arguments

• expression — A constant expression that returns a query string. Query parameters and scalar subqueries are allowed.
• SELECT ... — A SELECT query that returns a query string. This form is syntactic sugar for a scalar subquery: eval(SELECT ...) is equivalent to eval((SELECT ...)), so the query must return exactly one row and one column.

The value of the expression must have one of these types:

• String
• Nullable(String)
• LowCardinality(String)
• LowCardinality(Nullable(String))

If the value is NULL, eval throws an exception.

Returned Value

Returns the result of the generated SELECT query as a table.

The output schema is determined when the eval table function is analyzed, so outer queries can refer to the generated query’s real column names and types.

Examples

Evaluate a constant expression:

SELECT * FROM eval('SEL' || 'ECT 1 AS x');

Result:

┌─x─┐
│ 1 │
└───┘

Use a query parameter:

SET param_q = 'SELECT 2 AS y';
SELECT * FROM eval({q:String});

Result:

┌─y─┐
│ 2 │
└───┘

Evaluate an input SELECT query that returns query text:

SELECT * FROM eval(SELECT 'SELECT 3 AS z');

Result:

┌─z─┐
│ 3 │
└───┘

Use the generated schema in the outer query:

SELECT x + 1 FROM eval('SELECT 4 AS x');

Result:

┌─plus(x, 1)─┐
│          5 │
└────────────┘

Restrictions

• eval works only with query analyzer (enable_analyzer = 1).
• The generated query must be a single SELECT query without output options such as INTO OUTFILE or FORMAT. eval does not execute multiple statements.
• The generated query must be self-contained: it is resolved in its own scope and cannot reference WITH aliases or columns of the outer query.
• The generated query cannot use the eval table function again.
• eval cannot be used as an argument of another table function, such as remote('host', eval(...)). To execute the generated query on a remote server, wrap it into view: remote('host', view(SELECT * FROM eval(...))).
• The argument is evaluated once while the query is analyzed, not once per row or block.
• The outer query log records the original query that contains eval; the generated SELECT query is not logged as a separate user query.

Related

• view table function
• allow_experimental_eval_table_function setting