Partial indexes

Partial indexes allow you to improve query performance by reducing the index size. A smaller index is faster to scan, easier to maintain, and requires less storage.

Partial indexing works by specifying the rows defined by a conditional expression (called the predicate of the partial index), typically in the WHERE clause of the table.

Partial indexes can be UNIQUE. A UNIQUE partial index enforces the constraint that for each possible tuple of indexed columns, only one row that satisfies the index_predicate is allowed in the table.

A partial index might not be chosen even if the implication holds in cases where there are better query plans.

The logical implication holds if all sub-expressions of the index_predicate are present as is in the where_expression. For example, assume where_expression = A AND B AND C, index_predicate_1 = A AND B, index_predicate_2 = A AND B AND D, index_predicate_3 = A AND B AND C AND D. Then where_expression only implies index_predicate_1.

Currently, valid mathematical implications are not taken into account when checking for logical implication. For example, even if where_expression = x > 5 and index_predicate = x > 4, the SELECT query will not use the index for scanning. This is because the two sub-expressions x > 5 and x > 4 differ.

Syntax

CREATE INDEX index_name ON table_name(column_list) WHERE condition;

Example

Create a keyspace and a table as follows:

ycqlsh> CREATE KEYSPACE example;
ycqlsh> USE example;
ycqlsh:example> CREATE TABLE orders (customer_id INT,
                                    order_date TIMESTAMP,
                                    product JSONB,
                                    warehouse_id INT,
                                    amount DOUBLE,
                                    PRIMARY KEY ((customer_id), order_date))
                WITH transactions = { 'enabled' : true };

Create a partial index for the warehouse_id column with the expression WHERE warehouse_id < 100 to be able to enable a faster scanning of rows on queries which will benefit from such a search criteria.

ycqlsh:example> CREATE INDEX idx ON orders (warehouse_id)
                WHERE warehouse_id < 100;

When using a prepared statement, the logical implication check (to decide if a partial index is usable) will only consider those sub-expressions of where_expression that don't have dynamic parameters. This is because the query plan is decided before execution (when a statement is prepared).

ycqlsh:example> EXPLAIN SELECT product FROM orders
                WHERE warehouse_id < 100 AND order_date >= ?; // Idx can be used

 QUERY PLAN
------------------------------------------
 Index Scan using temp.idx on temp.orders
   Filter: (order_date >= :order_date)

ycqlsh:example> EXPLAIN SELECT product FROM orders
                WHERE warehouse_id < ? and order_date >= ?; // Idx cannot be used

 QUERY PLAN
--------------------------------------------------------------------------
 Seq Scan on temp.orders
   Filter: (warehouse_id < :warehouse_id) AND (order_date >= :order_date)

Partial indexes with combinations of operators

Without partial indexes, many combinations of operators together on the same column in a SELECT's where expression (for example, WHERE v1 != NULL and v1 = 5) are not allowed.

ycqlsh:example> EXPLAIN SELECT product FROM orders
                WHERE warehouse_id != NULL AND warehouse_id = ?;

SyntaxException: Invalid CQL Statement. Illogical condition for where clause
EXPLAIN SELECT product from orders where warehouse_id != NULL and warehouse_id = ?;
                                                                  ^^^^^^^^^^^^
 (ql error -12)

With a partial index that subsumes some clauses of the SELECT's where expression, then two or more operators that would not otherwise be supported together are supported.

ycqlsh:example> CREATE INDEX warehouse_idx ON orders (warehouse_id)
                WHERE warehouse_id != NULL;
ycqlsh:example> EXPLAIN SELECT product FROM orders
                WHERE warehouse_id != NULL AND warehouse_id = ?; // warehouse_idx can be used

 QUERY PLAN
----------------------------------------------------
 Index Scan using temp.warehouse_idx on temp.orders
   Key Conditions: (warehouse_id = :warehouse_id)

Learn more

• PARTIAL INDEX in the YCQL API documentation

• Partial index with JSONB column