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mergeTreeProjection Table Function

Represents the contents of some projection in MergeTree tables. It can be used for introspection.

Syntax

mergeTreeProjection(database, table, projection)

Arguments

ArgumentDescription
databaseThe database name to read projection from.
tableThe table name to read projection from.
projectionThe projection to read from.

Returned value

A table object with columns provided by given projection.

Usage Example

CREATE TABLE test
(
    `user_id` UInt64,
    `item_id` UInt64,
    PROJECTION order_by_item_id
    (
        SELECT _part_offset
        ORDER BY item_id
    )
)
ENGINE = MergeTree
ORDER BY user_id;

INSERT INTO test SELECT number, 100 - number FROM numbers(5);
SELECT *, _part_offset FROM mergeTreeProjection(currentDatabase(), test, order_by_item_id);
   ┌─item_id─┬─_parent_part_offset─┬─_part_offset─┐
1. │      96 │                   4 │            0 │
2. │      97 │                   3 │            1 │
3. │      98 │                   2 │            2 │
4. │      99 │                   1 │            3 │
5. │     100 │                   0 │            4 │
   └─────────┴─────────────────────┴──────────────┘
DESCRIBE mergeTreeProjection(currentDatabase(), test, order_by_item_id) SETTINGS describe_compact_output = 1;
   ┌─name────────────────┬─type───┐
1. │ item_id             │ UInt64 │
2. │ _parent_part_offset │ UInt64 │
   └─────────────────────┴────────┘