scylladb/docs/dev/reverse-reads.md

Reverse reads

A read is called reverse when it reads with reverse clustering order (compared to that of the schema). Example:

CREATE TABLE mytable (
    pk int,
    ck int,
    s int STATIC,
    v int,
    PRIMARY KEY (pk, ck)
) WITH
    CLUSTERING ORDER BY (ck ASC);

# Forward read (using table's native order)
SELECT * FROM mytable WHERE pk = 1;
# Explicit forward order
SELECT * FROM mytable WHERE pk = 1 ORDER BY ck ASC;

# Reverse read
SELECT * FROM mytable WHERE pk = 1 ORDER BY ck DESC;

If the table's native clustering order is DESC, then a read with ASC order is considered reverse.

Native format

The native format uses ordering equivalent to that of a table with reverse clustering format. Using mytable as an example, the native reverse format would be an identical table my_reverse_table, which uses CLUSTERING ORDER BY (ck DESC);. This allows middle layers in a read pipeline to just use a schema with reversed clustering order and process the reverse stream as normal.

Request

The query::partition_slice::options::reversed flag is set. Clustering ranges in both query::partition_slice::_row_ranges and query::specific_ranges::_ranges (query::partition_slice::_specific_ranges) are reversed: they are ordered in reverse. Example:

For the clustering keys (ASC order): ck1, ck2, ck3, ck4, ck5, ck6. A _row_ranges field of a slice might contain this:

[ck1, ck2], [ck4, ck5]

The reversed version would look like this:

[ck5, ck4], [ck2, ck1]

In addition to this, the schema is reversed on the replica, at the start of the read, so all the reverse-capable and intermediate readers in the stack get a reversed schema to work with.

Result

Results are ordered with the reversed clustering order with the bounds of range-tombstones swapped. For example given the following partition:

ps{pk1}, sr{}, cr{ck1}, rt{[ck2, ck4)}, cr{ck2}, cr{ck3}, cr{ck4}, ck{ck5}, pe{}

The native reverse version would look like this:

ps{pk1}, sr{}, cr{ck5}, cr{ck4}, rt{(ck4, ck2]}, cr{ck3}, cr{ck2}, ck{ck1}, pe{}

Legend:

• ps = partitions-tart
• sr = static-row
• cr = clustering-row
• rt = range-tombstone
• pe = partition-end