Support SELECT without a FROM clause

fdb-relational-core/src/main/java/com/apple/foundationdb/relational/recordlayer/query/visitors/QueryVisitor.java

@@ -30,7 +30,8 @@
 import com.apple.foundationdb.record.query.plan.cascades.expressions.UpdateExpression;
 import com.apple.foundationdb.record.query.plan.cascades.predicates.CompatibleTypeEvolutionPredicate;
 import com.apple.foundationdb.record.query.plan.cascades.typing.Type;
 import com.apple.foundationdb.record.query.plan.cascades.values.FieldValue;
+import com.apple.foundationdb.record.query.plan.cascades.values.LiteralValue;
 import com.apple.foundationdb.record.query.plan.cascades.values.Value;
 import com.apple.foundationdb.record.util.pair.NonnullPair;
 import com.apple.foundationdb.relational.api.exceptions.ErrorCode;
@@ -222,11 +223,20 @@
     @Nonnull
     @Override
     public LogicalOperator visitSimpleTable(@Nonnull RelationalParser.SimpleTableContext simpleTableContext) {
-        Assert.notNullUnchecked(simpleTableContext.fromClause(), ErrorCode.UNSUPPORTED_QUERY, "query is not supported");
         getDelegate().pushPlanFragment();
-        simpleTableContext.fromClause().accept(this);
+        if (simpleTableContext.fromClause() != null) {
+            simpleTableContext.fromClause().accept(this);
+        } else {
+            // No FROM clause: synthesize a single-row source so constant expressions
+            // can be projected over exactly one output row (standard SQL semantics).
+            final var dummyElement = Expression.fromUnderlying(LiteralValue.ofScalar(true));
+            final var arrayOfOne = getDelegate().resolveFunction("__internal_array", false, dummyElement);
+            final var explodeExpr = new ExplodeExpression(arrayOfOne.getUnderlying());
+            final var syntheticQuantifier = Quantifier.forEach(Reference.initialOf(explodeExpr));
+            getDelegate().getCurrentPlanFragment().setOperator(LogicalOperator.newUnnamedOperator(Expressions.empty(), syntheticQuantifier));
+        }
 
-        var where = Optional.ofNullable(simpleTableContext.fromClause().whereExpr() == null ?
+        var where = Optional.ofNullable(simpleTableContext.fromClause() == null || simpleTableContext.fromClause().whereExpr() == null ?
                 null :
                 visitWhereExpr(simpleTableContext.fromClause().whereExpr()));
 

yaml-tests/src/test/java/YamlIntegrationTests.java

@@ -493,4 +493,9 @@ public void recordTypeKeyTest(YamlTest.Runner runner) throws Exception {
     public void filterIndexTest(YamlTest.Runner runner) throws Exception {
         runner.runYamsql("filter-index.yamsql");
     }
+
+    @TestTemplate
+    public void selectWithoutFrom(YamlTest.Runner runner) throws Exception {
+        runner.runYamsql("select-without-from.yamsql");
+    }
 }

yaml-tests/src/test/resources/select-without-from.yamsql

@@ -0,0 +1,59 @@
+#
+# select-without-from.yamsql
+#
+# This source file is part of the FoundationDB open source project
+#
+# Copyright 2021-2025 Apple Inc. and the FoundationDB project authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+---
+options:
+    supported_version: !current_version
+---
+schema_template:
+    create table T(id bigint, primary key(id))
+---
+test_block:
+  preset: single_repetition_ordered
+  tests:
+    -
+      - query: SELECT 'bob' AS greeting
+      - resultMetadata: [{GREETING: STRING}]
+      - result: [{'bob'}]
+    -
+      - query: SELECT 42 AS answer
+      - resultMetadata: [{ANSWER: INTEGER}]
+      - result: [{42}]
+    -
+      - query: SELECT 1 + 1 AS result
+      - resultMetadata: [{RESULT: INTEGER}]
+      - result: [{2}]
+    -
+      - query: SELECT 'hello' AS a, 'world' AS b
+      - resultMetadata: [{A: STRING}, {B: STRING}]
+      - result: [{'hello', 'world'}]
+    -
+      - query: SELECT true AS flag
+      - resultMetadata: [{FLAG: BOOLEAN}]
+      - result: [{true}]
+    -
+      - query: SELECT 41
+      - resultMetadata: [{_0: INTEGER}]
+      - result: [{41}]
+    -
+      # WHERE without FROM is a syntax error: WHERE is part of fromClause in the grammar
+      # and cannot appear without a preceding FROM.
+      - query: SELECT 'bob' WHERE true
+      - error: "42601"
+...