SQLAlchemy query generation (lsst.daf.relation.sql)

Overview

The Engine provided by the lsst.daf.relation.sql package translates Relation trees to SQLAlchemy expressions. It doesn’t actually execute those expressions, however; running a SQL query and fetching its results back to a Python client is really best considered a Transfer between engines, with a SQL engine like this one as the source and an engine backed by in-memory Python objects (such as lsst.daf.relation.iteration) as its destination. Multi-engine relation trees can be executed by a Processor subclass.

This engine flattens back-to-back Join and Chain operations, and it actually reorders any combination of adjacent Join, Selection, Calculation, and Projection into a single SELECT...FROM...WHERE statement via the commutation rules for these operations. It assumes the database query optimizer will reorder at least these operations itself anyway, so the goal is to keep the query as simple as possible to stay out of its way and aid human readers.

The Engine.to_executable method transforms a Relation tree to a SQL SELECT or UNION thereof, as represented by SQLAlchemy_. The engine’s payload type is the Payload dataclass, which better maps to SQL tables or very simple subqueries that can be used like tables.

Support for custom UnaryOperation subclasses can be added implementing Engine.apply_custom_unary_operation.

Logical Columns

The Engine and Payload classes are generic over a parameter we refer to as the “logical column type”. In the simplest case, the logical column type is just sqlalchemy.sql.ColumnElement, and this is what the default implementations of most Engine methods assume. Custom subclasses of the SQL Engine can use other types, such as wrappers that hold one or more sqlalchemy.sql.ColumnElement objects, as long as they override a few Engine methods to handle them. This allows one column tag and logical column in the Relation representation of a query to map to multiple columns in the SQL representation.

Operation Ordering and Select

The SQL engine has a non-trivial Engine.conform method and overrides other methods to keep relation trees in a certain order when applying new operations. It uses a custom Select marker type to partition the tree into subqueries and mark a tree as conformed. This order attempts to avoid subqueries whenever possible by pulling Projection, Deduplication, Sort, and Slice operations downstream when commutation rules permit, both to keep the SQL output simple and to reduce the loss of row-ordering imposed by Sort operations whenever possible (i.e. a Sort can only be applied in the outermost query if it is to have any effect).

API reference

Classes

Engine(*, name, functions, _F] =, ...)	A concrete engine class for relations backed by a SQL database.
Payload(from_clause, where, ...)	A struct that represents a SQL table or simple SELECT query via SQLAlchemy objects.
Select(target[, payload])	A marker operation used by a the SQL engine to group relation trees into SELECT statements.