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4099833587
When a table has secondary indexes on *multiple* columns, and several
such columns are used for filtering in a query, Scylla chooses one
of these indexes as the main driver of the query, and the second
column's restriction is implemented as filtering.
Before this patch, the index to use was chosen fairly randomly, based on
the order of the indexes in the schema. This order may be different in
different coordinators, and may even change across restarts on the same
coordinators. This is not only inconsistent, it can cause outright wrong
results when using *paging* and switching (or restarting) coordinates
in the middle of a paged scan... One coordinator saves one index's key
in the paging state, and then the other coordinator gets this paging
state and wrongly believes it is supposed to be a key of a *different*
index.
The fix in this patch is to pick the index suitable for the first
indexed column mentioned in the query. This has two benefits over
the situation before the patch:
1. The decision of which index to use no longer changes between
coordinators or across restarts - it just depends on the schema
and the specific query.
2. Different indexes can have different "specificity" so using one
or the other can change the query's performance. After this patch,
the user is in control over which index is used by changing the
order of terms in the query. A curious user can use tracing to
check which index was used to implement a particular query.
An xfailing test we had for this issue no longer fails, so the "xfail"
marker is removed.
Fixes #7969
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
(cherry picked from commit 77c61f907e)
Closes scylladb/scylladb#18963