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f8aaeb5e87
ScyllaDB has special counter columns for which atomic add/subtract
operations like `SET a = a + 1` are allowed. Such operations have not
been allowed on ordinary non-counter columns, as they would not be
properly atomic - the read an the write are separate, and concurrent
operations can have incorrect results.
This patch makes it allowed to use such atomic add/subtract operations
in *LWT* statements. Some examples:
UPDATE ... SET a = a - 1 IF a > 0
UPDATE ... SET a = a + 1 IF EXISTS
UPDATE ... SET a = a + 1 a != NULL
The row updated in the operation, and the updated column (a) should
be initialized before the update - arithmetic operations on missing
column values silently leave the column null (no error is generated).
This add/subtract operations is allowed on any numeric column -
integer or floating point of any size.
The ability of LWT to fetch the old values of a column and use it to
calculate the new value has long been available in our internal CAS
implementation - and has been in use for years in Alternator - but until
this patch it was not exposed in CQL's LWT.
This patch does not add new syntax to CQL - the "SET a = a + b"
and "SET a = a - b" syntax that already existed for counters is now
allowed for non-counters.
This is a new Scylla-only feature that does not exist in Cassandra.
Fixes #10568
Signed-off-by: Nadav Har'El <nyh@scylladb.com>