Change _clustering_prefix_restrictions and _idx_tbl_ck_prefix
(the latter is the equivalent of the former, for indexed queries),
to use predicate instead of expressions. This lets us do
more of the work of solving restrictions during prepare time.
We only handle single-column restrictions here. Multi-column
restrictions use the existing path.
We introduce two helpers:
- value_set_to_singleton() converts a restriction solution to a singleton
when we know that's the only possible answer
- replace_column_def() overload for predicate, similar to the
existing overload for expressions
There is a wart in get_single_column_clustering_bounds(): we arrive at
his point with the two vectors possibly pointing at different
columns. Previously, possible_lhs_values() did this check while solving.
We now check for it here.
The predicate::on variant gets another member, for clustering key prefixes.
Since everything is still handled by the legacy paths, we mostly
error out.
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