CQL statements carry expressions in many contexts: the SELECT, WHERE, SET, and IF clauses, plus various attributes. Previously, each of these contexts had its own representation for an expression, and another one for the same expression but before preparation. We have been gradually moving towards a uniform representation of expressions.
This series tackles SELECT clause elements (selectors), in their unprepared phase. It's relatively simple since there are only five types of expression components (column references, writetime/ttl modifiers, function calls, casts, and field selections). Nevertheless, there isn't much commonality with previously converted expression elements so quite a lot of code is involved.
After the series, we are still left with a custom post-prepare representation of expressions. It's quite complicated since it deals with two passes, for aggregation, so it will be left for another series.
Closes#14219
* github.com:scylladb/scylladb:
cql3: seletor: drop inheritance from assignment_testable
cql3: selection: rely on prepared expressions
cql3: selection: prepare selector expressions
cql3: expr: match counter arguments to function parameters expecting bigint
cql3: expr: avoid function constant-folding if a thread is needed
cql3: add optional type annotation to assignment_testable
cql3: expr: wire unresolved_identifier to test_assignment()
cql3: expr: support preparing column_mutation_attribute
cql3: expr: support preparing SQL-style casts
cql3: expr: support preparing field_selection expressions
cql3: expr: make the two styles of cast expressions explicit
cql3: error injection functions: mark enabled_injections() as impure
cql3: eliminate dynamic_cast<selector> from functions::get()
cql3: test_assignment: pass optional schema everywhere
cql3: expr: prepare_expr(): allow aggregate functions
cql3: add checks for aggregation functions after prepare
cql3: expr: add verify_no_aggregate_functions() helper
test: add regression test for rejection of aggregates in the WHERE clause
cql3: expr: extract column_mutation_attribute_type
cql3: expr: add fmt formatter for column_mutation_attribute_kind
cql3: statements: select_statement: reuse to_selectable() computation in SELECT JSON
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