system.hypothetical_indexes

System table listing hypothetical (what-if) indexes defined in the current session

Lists every hypothetical (what-if) skip index defined in the current session. See CREATE HYPOTHETICAL INDEX and EXPLAIN WHATIF.

The contents are session-scoped: each connection sees only its own hypothetical indexes, and the table is empty when no indexes have been created in the current session.

The current (database, table) are resolved by UUID at query time, so they reflect RENAME TABLE and entries for dropped tables are hidden automatically.

Columns

Column	Type	Description
database	String	Target database.
table	String	Target table.
name	String	Index name.
type	String	Index type (minmax, set, bloom_filter, etc.).
type_full	String	Index type expression including arguments, e.g. bloom_filter(0.01).
expression	String	Index expression as written in CREATE HYPOTHETICAL INDEX.
granularity	UInt64	Number of data granules per index granule.

Example

CREATE HYPOTHETICAL INDEX i1 ON t (b) TYPE bloom_filter(0.01)  GRANULARITY 1;
CREATE HYPOTHETICAL INDEX i2 ON t (b) TYPE bloom_filter(0.001) GRANULARITY 1;

SELECT database, table, name, type, type_full, expression, granularity
FROM system.hypothetical_indexes;

┌─database─┬─table─┬─name─┬─type─────────┬─type_full───────────┬─expression─┬─granularity─┐
│ default  │ t     │ i1   │ bloom_filter │ bloom_filter(0.01)  │ b          │           1 │
│ default  │ t     │ i2   │ bloom_filter │ bloom_filter(0.001) │ b          │           1 │
└──────────┴───────┴──────┴──────────────┴─────────────────────┴────────────┴─────────────┘

type is the base type name and type_full includes the arguments, so users can distinguish between parametrized variants like bloom_filter(0.01) and bloom_filter(0.001).

See also

• CREATE HYPOTHETICAL INDEX
• EXPLAIN WHATIF