Until this patch, dropping columns from a table was completely forbidden
if this table has any materialized views or secondary indexes. However,
this is excessively harsh, and not compatible with Cassandra which does
allow dropping columns from a base table which has a secondary index on
*other* columns. This incompatibility was raised in the following
Stackoverflow question:
https://stackoverflow.com/questions/55757273/error-while-dropping-column-from-a-table-with-secondary-index-scylladb/55776490
In this patch, we allow dropping a base table column if none of its
materialized views *needs* this column. Columns selected by a view
(as regular or key columns) are needed by it, of course, but when
virtual columns are used (namely, there is a view with same key columns
as the base), *all* columns are needed by the view, so unfortunately none
of the columns may be dropped.
After this patch, when a base-table column cannot be dropped because one
of the materialized views needs it, the error message will look like:
exceptions::invalid_request_exception: Cannot drop column a from base
table ks.cf: a materialized view cf_a_idx_index needs this column.
This patch also includes extensive testing for the cases where dropping
columns are now allowed, and not allowed. The secondary-index tests are
especially interesting, because they demonstrate that now usually (when
a non-key column is being indexed) dropping columns will be allowed,
which is what originally bothered the Stackoverflow user.
Fixes#4448.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190429214805.2972-1-nyh@scylladb.com>
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