ESQL: Make LOOKUP more left-joiny (elastic#118889) (elastic#119475) (elastic#120232)

* ESQL: Compute infrastruture for LEFT JOIN (elastic#118889)

This adds some infrastructure that we can use to run LOOKUP JOIN using
real LEFT JOIN semantics.

Right now if LOOKUP JOIN matches many rows in the `lookup` index we
merge all of the values into a multivalued field. So the number of rows
emitted from LOOKUP JOIN is the same as the number of rows that comes
into LOOKUP JOIN.

This change builds the infrastructure to emit one row per match, mostly
reusing the infrastructure from ENRICH.

* ESQL: Make LOOKUP more left-joiny (elastic#119475)

This makes `LOOKUP` return multiple rows if there are multiple matches. This is the way SQL works so it's *probably* what folks will expect. Even if it isn't, it allows for more optimizations. Like, this change doesn't optimize anything - it just changes the behavior. But there are optimizations you can do *later* that are transparent when we have *this* behavior, but not with the old behavior.

Example:
```
-  2  | [German, German, German] | [Austria, Germany, Switzerland]
+  2  | German                   | [Austria, Germany]
+  2  | German                   | Switzerland
+  2  | German                   | null
```

Relates: elastic#118781

Author: nik9000

server/src/main/java/org/elasticsearch/action/admin/cluster/node/capabilities/NodeCapability.java

@@ -41,4 +41,9 @@ public void writeTo(StreamOutput out) throws IOException {
 
         out.writeBoolean(supported);
     }
+
+    @Override
+    public String toString() {
+        return "NodeCapability{supported=" + supported + '}';
+    }
 }

x-pack/plugin/esql/compute/src/main/generated-src/org/elasticsearch/compute/data/BooleanArrayBlock.java

@@ -212,10 +212,46 @@ default boolean mvSortedAscending() {
     /**
      * Expand multivalued fields into one row per value. Returns the same block if there aren't any multivalued
      * fields to expand. The returned block needs to be closed by the caller to release the block's resources.
-     * TODO: pass BlockFactory
      */
     Block expand();
 
+    /**
+     * Build a {@link Block} with a {@code null} inserted {@code before} each
+     * listed position.
+     * <p>
+     *     Note: {@code before} must be non-decreasing.
+     * </p>
+     */
+    default Block insertNulls(IntVector before) {
+        // TODO remove default and scatter to implementation where it can be a lot more efficient
+        int myCount = getPositionCount();
+        int beforeCount = before.getPositionCount();
+        try (Builder builder = elementType().newBlockBuilder(myCount + beforeCount, blockFactory())) {
+            int beforeP = 0;
+            int nextNull = before.getInt(beforeP);
+            for (int mainP = 0; mainP < myCount; mainP++) {
+                while (mainP == nextNull) {
+                    builder.appendNull();
+                    beforeP++;
+                    if (beforeP >= beforeCount) {
+                        builder.copyFrom(this, mainP, myCount);
+                        return builder.build();
+                    }
+                    nextNull = before.getInt(beforeP);
+                }
+                // This line right below this is the super inefficient one.
+                builder.copyFrom(this, mainP, mainP + 1);
+            }
+            assert nextNull == myCount;
+            while (beforeP < beforeCount) {
+                nextNull = before.getInt(beforeP++);
+                assert nextNull == myCount;
+                builder.appendNull();
+            }
+            return builder.build();
+        }
+    }
+
     /**
      * Builds {@link Block}s. Typically, you use one of it's direct supinterfaces like {@link IntBlock.Builder}.
      * This is {@link Releasable} and should be released after building the block or if building the block fails.

x-pack/plugin/esql/compute/src/main/generated-src/org/elasticsearch/compute/data/BytesRefArrayBlock.java

@@ -246,4 +246,9 @@ public OrdinalBytesRefBlock expand() {
     public long ramBytesUsed() {
         return ordinals.ramBytesUsed() + bytes.ramBytesUsed();
     }
+
+    @Override
+    public String toString() {
+        return getClass().getSimpleName() + "[ordinals=" + ordinals + ", bytes=" + bytes + "]";
+    }
 }

x-pack/plugin/esql/compute/src/main/generated-src/org/elasticsearch/compute/data/DoubleArrayBlock.java

@@ -149,7 +149,7 @@ $endif$
                 int valueCount = getValueCount(pos);
                 int first = getFirstValueIndex(pos);
                 if (valueCount == 1) {
-                    builder.append$Type$(get$Type$(getFirstValueIndex(pos)$if(BytesRef)$, scratch$endif$));
+                    builder.append$Type$(get$Type$(first$if(BytesRef)$, scratch$endif$));
                 } else {
                     builder.beginPositionEntry();
                     for (int c = 0; c < valueCount; c++) {