Add javadocs and small improvements to join code.

processing/src/main/java/org/apache/druid/segment/ColumnProcessorFactory.java

@@ -41,6 +41,9 @@ public interface ColumnProcessorFactory<T>
 {
   /**
    * This default type will be used when the underlying column has an unknown type.
+   *
+   * This allows a column processor factory to specify what type it prefers to deal with (the most 'natural' type for
+   * whatever it is doing) when all else is equal.
    */
   ValueType defaultType();
 

processing/src/main/java/org/apache/druid/segment/join/HashJoinEngine.java

@@ -45,8 +45,11 @@ private HashJoinEngine()
    * joinable clause's prefix (see {@link JoinableClause#getPrefix()}) will come from the Joinable's column selector
    * factory, and all other columns will come from the leftCursor's column selector factory.
    *
-   * Ensuing that the joinable clause's prefix does not conflict with any columns from "leftCursor" is the
-   * responsibility of the caller.
+   * Ensuring that the joinable clause's prefix does not conflict with any columns from "leftCursor" is the
+   * responsibility of the caller. If there is such a conflict (for example, if the joinable clause's prefix is "j.",
+   * and the leftCursor has a field named "j.j.abrams"), then the field from the leftCursor will be shadowed and will
+   * not be queryable through the returned Cursor. This happens even if the right-hand joinable doesn't actually have a
+   * column with this name.
    */
   public static Cursor makeJoinCursor(final Cursor leftCursor, final JoinableClause joinableClause)
   {

processing/src/main/java/org/apache/druid/segment/join/JoinConditionAnalysis.java

@@ -62,6 +62,14 @@ private JoinConditionAnalysis(
     this.nonEquiConditions = nonEquiConditions;
   }
 
+  /**
+   * Analyze a join condition.
+   *
+   * @param condition   the condition expression
+   * @param rightPrefix prefix for the right-hand side of the join; will be used to determine which identifiers in
+   *                    the condition come from the right-hand side and which come from the left-hand side
+   * @param macroTable  macro table for parsing the condition expression
+   */
   public static JoinConditionAnalysis forExpression(
       final String condition,
       final String rightPrefix,

processing/src/main/java/org/apache/druid/segment/join/JoinableClause.java

@@ -48,7 +48,15 @@ public JoinableClause(@Nullable String prefix, Joinable joinable, JoinType joinT
   }
 
   /**
-   * The prefix to apply to all columns from the Joinable.
+   * The prefix to apply to all columns from the Joinable. The idea is that during a join, any columns that start with
+   * this prefix should be retrieved from our Joinable's {@link JoinMatcher#getColumnSelectorFactory()}. Any other
+   * columns should be returned from the left-hand side of the join.
+   *
+   * The prefix can be any string, as long as it is nonempty and not itself a prefix of the reserved column name
+   * {@code __time}.
+   *
+   * @see #getAvailableColumnsPrefixed() the list of columns from our {@link Joinable} with prefixes attached
+   * @see #unprefix a method for removing prefixes
    */
   public String getPrefix()
   {

processing/src/main/java/org/apache/druid/segment/join/PossiblyNullColumnValueSelector.java

@@ -26,6 +26,10 @@
 import javax.annotation.Nullable;
 import java.util.function.BooleanSupplier;
 
+/**
+ * A {@link ColumnValueSelector} that wraps a base selector but might also generate null values on demand. This
+ * is used for "righty" joins (see {@link JoinType#isRighty()}), which may need to generate nulls on the left-hand side.
+ */
 public class PossiblyNullColumnValueSelector<T> implements ColumnValueSelector<T>
 {
   private final ColumnValueSelector<T> baseSelector;

processing/src/main/java/org/apache/druid/segment/join/table/IndexedTable.java

@@ -26,27 +26,67 @@
 import java.util.List;
 import java.util.Map;
 
+/**
+ * An interface to a table where some columns (the 'key columns') have indexes that enable fast lookups.
+ *
+ * The main user of this class is {@link IndexedTableJoinable}, and its main purpose is to participate in joins.
+ */
 public interface IndexedTable
 {
+  /**
+   * Returns the columns of this table that have indexes.
+   */
   List<String> keyColumns();
 
+  /**
+   * Returns all columns of this table, including the key and non-key columns.
+   */
   List<String> allColumns();
 
+  /**
+   * Returns the signature of this table: a map where each key is a column from {@link #allColumns()} and each value
+   * is a type code.
+   */
   Map<String, ValueType> rowSignature();
 
+  /**
+   * Returns the number of rows in this table. It must not change over time, since it is used for things like algorithm
+   * selection and reporting of cardinality metadata.
+   */
   int numRows();
 
+  /**
+   * Returns the index for a particular column. The provided column number must be that column's position in
+   * {@link #allColumns()}.
+   */
   Index columnIndex(int column);
 
+  /**
+   * Returns a reader for a particular column. The provided column number must be that column's position in
+   * {@link #allColumns()}.
+   */
   Reader columnReader(int column);
 
+  /**
+   * Indexes support fast lookups on key columns.
+   */
   interface Index
   {
+    /**
+     * Returns the list of row numbers where the column this Reader is based on contains 'key'.
+     */
     IntList find(Object key);
   }
 
+  /**
+   * Readers support reading values out of any column.
+   */
   interface Reader
   {
+    /**
+     * Read the value at a particular row number. Throws an exception if the row is out of bounds (must be between zero
+     * and {@link #numRows()}).
+     */
     @Nullable
     Object read(int row);
   }

processing/src/main/java/org/apache/druid/segment/join/table/IndexedTableJoinMatcher.java

@@ -75,7 +75,7 @@ public class IndexedTableJoinMatcher implements JoinMatcher
     if (condition.isAlwaysTrue()) {
       this.conditionMatchers = Collections.singletonList(() -> IntIterators.fromTo(0, table.numRows()));
     } else if (condition.isAlwaysFalse()) {
-      this.conditionMatchers = Collections.singletonList(() -> IntIterators.fromTo(0, 0));
+      this.conditionMatchers = Collections.singletonList(() -> IntIterators.EMPTY_ITERATOR);
     } else if (condition.getNonEquiConditions().isEmpty()) {
       this.conditionMatchers = condition.getEquiConditions()
                                         .stream()