Package com.apple.foundationdb.record.query.plan.temp

Class AliasMap

  • public class AliasMap
extends Object
    This class encapsulates mappings between CorrelationIdentifiers and helpers to create and maintain these mappings. While various operations or in general algorithms traverse the query graph(s) it is important to keep track of what alias (of a quantifier) should be considered to what other alias (of another quantifier). A quantifier (Quantifier is referred to via CorrelationIdentifier. In this way there are no direct pointers to objects which is desirable while the graph is mutated through transformations. At various points during the planning of a query we may want to determine e.g. if a sub-graph would produce the same result as some other sub-graph. Similarly, we may want to determine whether a sub-graph is already properly memoized or not. In general, we need to determine if and how two sub-graphs are related to each other (usually equality but also subsumption, etc.) All of that is rather straightforward if both sub-graphs are completely uncorrelated and therefore independent of each other, or if they are correlated to a mutually external parameter. In these cases the AliasMap implemented here would effectively be trivial, i.e. the map would be conceptually mapping identifiers to themselves. There are cases, however, that need some care to establish more complex relationships such as those mentioned above. Example 1: sub graph 1 
        
        SELECT T.* FROM T WHERE T.x = c1
    sub graph 2 
        
        SELECT T.* FROM T WHERE T.x = c2
    We wold like to establish, if these graphs are equal or not. Both of them are not complete; that is, we wouldn't be able to execute them as-is. The graphs can only be equal with respect to a particular relationship between c1 and c2. 
        
            SELECT * FROM R AS c1, (SELECT T.* FROM T WHERE T.x = c1)
    is the same as 
        
            SELECT * FROM R AS c2, (SELECT T.* FROM T WHERE T.x = c2)
    whereas 
        
            SELECT * FROM R AS c1, (SELECT T.* FROM T WHERE T.x = c1)
    is definitely not the same as 
        
            SELECT * FROM S AS c2, (SELECT T.* FROM T WHERE T.x = c2)
    In short, a sub-graph can only be considered equal to another sub-graph if we assume an equality over their correlated references. If we view a correlated expression as a function of its correlations, then two sub-graph are equivalent if and only if they are identically equal (i.e., equal on their entire domain) when viewed as functions. Example 2: sub graph 1 
        
        SELECT T.* FROM T WHERE T.x = c1 AND T.y = c2
    sub graph 2 
        
        SELECT T.* FROM T WHERE T.x = c2 AND T.y = c1
    This scenario is somewhat more complicated as it offers more degrees of freedom. 
        
            SELECT * FROM R AS c1, S AS c2, (SELECT T.* FROM T WHERE T.x = c1 AND T.y = c2)
    is the same as 
        
            SELECT * FROM R AS c2, S AS c1, (SELECT T.* FROM T WHERE T.x = c2 AND T.y = c1)
    This observation is identical to the one in example 1. Due to the commutative nature we also would like to be able to realize that even this is the same: 
        
    SELECT * FROM R AS c1, S AS c2, (SELECT T.* FROM T WHERE T.x = c2 AND T.y = c1)
    In any case we need to keep track of what's currently considered bound and equal while we traverse the query graph. When we descend into correlated sub-graphs during an algorithm while we e.g. compute a function like Correlated.semanticEquals(java.lang.Object, com.apple.foundationdb.record.query.plan.temp.AliasMap) we need to have all outside references bound and mapped in some way: should we treat c1 on the left as c2 on the right or the other way around? It is the job of an object of this class to facilitate the construction and the lookup of these mappings. Algorithms walking the graph make use of this map, construct and reconstruct assumed equality while traversing variation (such as permutations) of possible mappings. This class is immutable, all perceived "mutations" cause a new object to be created.