ESQL: Pragma to load from stored fields (#122891)

This creates a `pragma` you can use to request that fields load from a
stored field rather than doc values. It implements that pragma for
`keyword` and number fields.

We expect that, for some disk configuration and some number of fields,
that it's faster to load those fields from _source or stored fields than
it is to use doc values. Our default is doc values and on my laptop it's
*always* faster to use doc values. But we don't ship my laptop to every
cluster.

This will let us experiment and debug slow queries by trying to load
fields a different way.

You access this pragma with:
```
curl -HContent-Type:application/json -XPOST localhost:9200/_query?pretty -d '{
    "query": "FROM foo",
    "pragma": {
        "field_extract_preference": "STORED"
    }
}'
```

On a release build you'll need to add `"accept_pragma_risks": true`.

Author: nik9000

docs/changelog/122891.yaml

@@ -0,0 +1,5 @@
+pr: 122891
+summary: Pragma to load from stored fields
+area: ES|QL
+type: enhancement
+issues: []

modules/mapper-extras/src/main/java/org/elasticsearch/index/mapper/extras/ScaledFloatFieldMapper.java

@@ -320,7 +320,7 @@ public BlockLoader blockLoader(BlockLoaderContext blContext) {
                 // Counters are not supported by ESQL so we load them in null
                 return BlockLoader.CONSTANT_NULLS;
             }
-            if (hasDocValues()) {
+            if (hasDocValues() && (blContext.fieldExtractPreference() != FieldExtractPreference.STORED || isSyntheticSource)) {
                 return new BlockDocValuesReader.DoublesBlockLoader(name(), l -> l / scalingFactor);
             }
             if (isSyntheticSource) {
@@ -333,7 +333,8 @@ public Builder builder(BlockFactory factory, int expectedCount) {
             }
 
             ValueFetcher valueFetcher = sourceValueFetcher(blContext.sourcePaths(name()));
-            BlockSourceReader.LeafIteratorLookup lookup = isStored() || isIndexed()
+            BlockSourceReader.LeafIteratorLookup lookup = hasDocValues() == false && (isStored() || isIndexed())
+                // We only write the field names field if there aren't doc values or norms
                 ? BlockSourceReader.lookupFromFieldNames(blContext.fieldNames(), name())
                 : BlockSourceReader.lookupMatchingAll();
             return new BlockSourceReader.DoublesBlockLoader(valueFetcher, lookup);

modules/mapper-extras/src/test/java/org/elasticsearch/index/mapper/extras/ScaledFloatFieldBlockLoaderTests.java

@@ -18,8 +18,8 @@
 import java.util.Map;
 
 public class ScaledFloatFieldBlockLoaderTests extends NumberFieldBlockLoaderTestCase<Double> {
-    public ScaledFloatFieldBlockLoaderTests() {
-        super(FieldType.SCALED_FLOAT);
+    public ScaledFloatFieldBlockLoaderTests(Params params) {
+        super(FieldType.SCALED_FLOAT, params);
     }
 
     @Override

server/src/main/java/org/elasticsearch/index/mapper/KeywordFieldMapper.java

@@ -748,7 +748,7 @@ NamedAnalyzer normalizer() {
 
         @Override
         public BlockLoader blockLoader(BlockLoaderContext blContext) {
-            if (hasDocValues()) {
+            if (hasDocValues() && (blContext.fieldExtractPreference() != FieldExtractPreference.STORED || isSyntheticSource)) {
                 return new BlockDocValuesReader.BytesRefsFromOrdsBlockLoader(name());
             }
             if (isStored()) {
@@ -806,7 +806,8 @@ private BlockSourceReader.LeafIteratorLookup sourceBlockLoaderLookup(BlockLoader
             if (getTextSearchInfo().hasNorms()) {
                 return BlockSourceReader.lookupFromNorms(name());
             }
-            if (isIndexed() || isStored()) {
+            if (hasDocValues() == false && (isIndexed() || isStored())) {
+                // We only write the field names field if there aren't doc values or norms
                 return BlockSourceReader.lookupFromFieldNames(blContext.fieldNames(), name());
             }
             return BlockSourceReader.lookupMatchingAll();

server/src/main/java/org/elasticsearch/index/mapper/MappedFieldType.java

@@ -692,8 +692,13 @@ public enum FieldExtractPreference {
         /**
          * No preference. Leave the choice of where to load the field from up to the FieldType.
          */
-        NONE;
-
+        NONE,
+        /**
+         * Prefer loading from stored fields like {@code _source} because we're
+         * loading many fields. The {@link MappedFieldType} can chose a different
+         * method to load the field if it needs to.
+         */
+        STORED;
     }
 
     /**

server/src/main/java/org/elasticsearch/index/mapper/NumberFieldMapper.java

@@ -1912,15 +1912,16 @@ public Function<byte[], Number> pointReaderIfPossible() {
 
         @Override
         public BlockLoader blockLoader(BlockLoaderContext blContext) {
-            if (hasDocValues()) {
+            if (hasDocValues() && (blContext.fieldExtractPreference() != FieldExtractPreference.STORED || isSyntheticSource)) {
                 return type.blockLoaderFromDocValues(name());
             }
 
             if (isSyntheticSource) {
                 return type.blockLoaderFromFallbackSyntheticSource(name(), nullValue, coerce);
             }
 
-            BlockSourceReader.LeafIteratorLookup lookup = isStored() || isIndexed()
+            BlockSourceReader.LeafIteratorLookup lookup = hasDocValues() == false && (isStored() || isIndexed())
+                // We only write the field names field if there aren't doc values or norms
                 ? BlockSourceReader.lookupFromFieldNames(blContext.fieldNames(), name())
                 : BlockSourceReader.lookupMatchingAll();
             return type.blockLoaderFromSource(sourceValueFetcher(blContext.sourcePaths(name())), lookup);

server/src/test/java/org/elasticsearch/index/mapper/blockloader/BooleanFieldBlockLoaderTests.java

@@ -17,13 +17,13 @@
 import java.util.Objects;
 
 public class BooleanFieldBlockLoaderTests extends BlockLoaderTestCase {
-    public BooleanFieldBlockLoaderTests() {
-        super(FieldType.BOOLEAN);
+    public BooleanFieldBlockLoaderTests(Params params) {
+        super(FieldType.BOOLEAN, params);
     }
 
     @Override
     @SuppressWarnings("unchecked")
-    protected Object expected(Map<String, Object> fieldMapping, Object value, boolean syntheticSource) {
+    protected Object expected(Map<String, Object> fieldMapping, Object value) {
         var nullValue = switch (fieldMapping.get("null_value")) {
             case Boolean b -> b;
             case String s -> Boolean.parseBoolean(s);

server/src/test/java/org/elasticsearch/index/mapper/blockloader/ByteFieldBlockLoaderTests.java

@@ -15,8 +15,8 @@
 import java.util.Map;
 
 public class ByteFieldBlockLoaderTests extends NumberFieldBlockLoaderTestCase<Integer> {
-    public ByteFieldBlockLoaderTests() {
-        super(FieldType.BYTE);
+    public ByteFieldBlockLoaderTests(Params params) {
+        super(FieldType.BYTE, params);
     }
 
     @Override

server/src/test/java/org/elasticsearch/index/mapper/blockloader/DateFieldBlockLoaderTests.java

@@ -23,13 +23,13 @@
 import java.util.Objects;
 
 public class DateFieldBlockLoaderTests extends BlockLoaderTestCase {
-    public DateFieldBlockLoaderTests() {
-        super(FieldType.DATE);
+    public DateFieldBlockLoaderTests(Params params) {
+        super(FieldType.DATE, params);
     }
 
     @Override
     @SuppressWarnings("unchecked")
-    protected Object expected(Map<String, Object> fieldMapping, Object value, boolean syntheticSource) {
+    protected Object expected(Map<String, Object> fieldMapping, Object value) {
         var format = (String) fieldMapping.get("format");
         var nullValue = fieldMapping.get("null_value") != null ? format(fieldMapping.get("null_value"), format) : null;
 

server/src/test/java/org/elasticsearch/index/mapper/blockloader/DoubleFieldBlockLoaderTests.java

@@ -15,8 +15,8 @@
 import java.util.Map;
 
 public class DoubleFieldBlockLoaderTests extends NumberFieldBlockLoaderTestCase<Double> {
-    public DoubleFieldBlockLoaderTests() {
-        super(FieldType.DOUBLE);
+    public DoubleFieldBlockLoaderTests(Params params) {
+        super(FieldType.DOUBLE, params);
     }
 
     @Override