Red — sql-tail
SQL-backed tail
Status: implemented; regression coverage is in t/89-sql-tail.rakutest.
Behavior
tail(n)returns a lazy ResultSeq containing the last n elements in their original order.Honor explicit sorting. Without sorting, use the primary key in ascending order, including composite primary keys.
tail()executes the query and returns the last projected value or model, preserving the current empty-query behavior.Without an explicit order or available primary key, retain the in-memory behavior. Throw a typed exception when
$*RED-FALLBACK = False.Keep inherited forms such as Callable arguments in memory.
Implementation
Represent ordering expressions and directions separately. Normalize the existing negative-expression convention for descending order.
Preserve row identity as internal metadata through projections. Export keys across SELECT boundaries as hidden fields, without adding them to results.
Aggregates, DISTINCT, and grouping invalidate the original row identity; require explicit ordering or use the fallback for these queries.
Reverse every ordering component, apply LIMIT n, wrap the selection, and restore the original order in the outer SELECT.
Preserve any preceding head/skip selection in a separate boundary before applying tail.
Fuse subsequent safe maps and filters into the outer SELECT. Never push those filters before the selection of the last elements.
Preserve converters, types, unique aliases, bind ordering, and independent sibling queries. Export ordering values without ASC/DESC in selected fields.
Example:
SELECT last_rows.data_1 * 10 AS bla
FROM (
SELECT bla.bla AS data_1
FROM bla
WHERE bla.bla > 1
ORDER BY bla.bla DESC
LIMIT 7
) AS last_rows
WHERE last_rows.data_1 * 10 < 100
AND last_rows.data_1 * 10 <= 50
ORDER BY last_rows.data_1 ASCVerification
Cover the complete example, returning [40, 50], with a lazy ResultSeq and exactly one SQL execution.
Cover models, scalars, and containers; ascending, descending, and composite ordering; implicit simple/composite primary keys; and NULLs in both drivers.
Cover tail(0), empty queries, oversized counts, tail(), repeated tails, head/skip combinations, and subsequent filters and sorting.
Verify hidden fields, converters, binds, fallback without keys, and typed rejection when fallback is disabled.
Run the full SQLite suite and relevant PostgreSQL tests. Update the cookbook and Changes in English.
Defaults
Introduce implicit primary-key ordering only when tail needs it, without changing the global ordering of model queries.
Do not append automatic tie-breakers to explicit ordering. Include primary keys explicitly when deterministic ordering of ties is required.
Respect each driver's NULL ordering when reversing and restoring directions.