LateralColumnAliasRegistryImpl is a utility class that contains structures required for lateral column alias resolution. Here we store:

• currentAttributeDependencyLevelStack - Current attribute dependency level in the scope. Dependency level is defined as a maximum dependency in that attribute's expression tree. For example, in a query like:

SELECT a, b, a + b AS c, a + c AS d

Dependency levels will be as follows: level 0: a, b level 1: c level 2: d

We add a new entry to the stack for each new Alias resolution. This is needed because we can have nesting Aliases in the plan, that do not belong to the same LCA scope. For example, in the following query:

SELECT STRUCT('alpha' AS A, 'beta' AS B) ST

ST, A and B would be aliases in the same expression tree, but they do not belong in the same LCA scope.

• availableAttributes - All attributes that can be laterally referenced. This map is indexed by name, but contains a list of attributes with the same name. This is because it is possible to have multiple attributes with the same name in the scope, but they can't be laterally referenced. Handling ambiguous references is done in the getAttribute method. For the following query:

SELECT 0 AS a, 1 AS b, 2 AS c, b AS d, a AS e, d AS f, a AS g, g AS h, h AS i

availableAttributes will be: {a, b, c, d, e, f, g, h, i}

• referencedAliases - Aliases that have been laterally referenced. For the given query example, referencedAliases will be: {a, b, d, g, h}
• aliasDependencyLevels - Dependency levels of all aliases, indexed by dependency level. For the given query example, dependency levels will be as follows:

level 0: a, b, c level 1: d, e, g level 2: f, h level 3: i