Bind variables in CQL have two formats: positional (`?`) where a variable is referred to by its relative position in the statement, and named (`:var`), where the user is expected to supply a name->value mapping.
In 19a6e69001 we identified the case where a named bind variable appears twice in a query, and collapsed it to a single entry in the statement metadata. Without this, a driver using the named variable syntax cannot disambiguate which variable is referred to.
However, it turns out that users can use the positional call form even with the named variable syntax, by using the positional API of the driver. To support this use case, we add a configuration variable to disable the same-variable detection.
Because the detection has to happen when the entire statement is visible, we have to supply the configuration to the parser. We call it the `dialect` and pass it from all callers. The alternative would be to add a pre-prepare call similar to fill_prepare_context that rewrites all expressions in a statement to deduplicate variables.
A unit test is added.
Fixes#15559
This may be useful to users transitioning from Cassandra, so merits a backport.
Closesscylladb/scylladb#19493
* github.com:scylladb/scylladb:
cql3: add option to not unify bind variables with the same name
cql3: introduce dialect infrastructure
cql3: prepared_statement_cache: drop cache key default constructor
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