When presented with queries that use the same named bind variables twice, like this one:
```cql
SELECT p FROM table WHERE p = :x AND c = :x
```
Scylla generated empty `partition_key_bind_indexes` (`pk_indexes`).
`pk_indexes` tell the driver which bind variables it should use to calculate the partition token for a query. Without it, the driver is unable to determine the token and it will send the query to a random node.
Scylla should generate pk_indexes which tell the driver that it can use bind variable with `bind_index = 0` to calculate the partition token for this query.
The problem was that `_target_columns` keep only a single target_column for each bind variable.
In the example above `:x` is compared with both `p` and `c`, but `_target_columns` would contain only one of them, and Scylla wasn't able to tell that this bind variable is compared with a partition key column.
To fix it, let's replace `_target_columns` with `_targets`. `_targets` keeps all comparisons
between bind variables and other expressions, so none of them will be forgotten/overwritten.
A `cql-pytest` reproducer is added.
I also added some comments in `prepare_context.hh/cc` to make it easier to read.
Fixes: https://github.com/scylladb/scylladb/issues/15374Closesscylladb/scylladb#15526
* github.com:scylladb/scylladb:
cql-pytest/test-prepare: remove xfail marker from *pk_indexes_duplicate_named_variables
cql3/prepare_context: fix generating pk_indexes for duplicate named bind variables
cql3: improve readability of prepare_context
cql-pytest: test generation of pk indexes during PREPARE
301b0a989a