fix: Querying empty Postgres table with 'time' dimension in a cube results in null value

Fixes #639

Author: paveltiunov

packages/cubejs-schema-compiler/test/PreAggregationsTest.js

@@ -204,6 +204,19 @@ describe('PreAggregations', function test() {
             incremental: true,
             updateWindow: '1 day'
           }
+        },
+        emptyPartitioned: {
+          type: 'rollup',
+          measureReferences: [count],
+          timeDimensionReference: EmptyHourVisitors.createdAt,
+          granularity: 'hour',
+          partitionGranularity: 'month',
+          scheduledRefresh: true,
+          refreshKey: {
+            every: '1 hour',
+            incremental: true,
+            updateWindow: '1 day'
+          }
         }
       }
     })
@@ -245,6 +258,11 @@ describe('PreAggregations', function test() {
         }
       }
     })
+    
+    cube('EmptyHourVisitors', {
+      extends: EveryHourVisitors,
+      sql: \`select v.* from \${visitors.sql()} v where created_at < '2000-01-01'\`
+    })
     `);
 
   function replaceTableName(query, preAggregation, suffix) {
@@ -536,6 +554,40 @@ describe('PreAggregations', function test() {
     });
   });
 
+  it('empty scheduled refresh', () => {
+    return compiler.compile().then(async () => {
+      const query = new PostgresQuery({ joinGraph, cubeEvaluator, compiler }, {
+        measures: [
+          'visitor_checkins.count'
+        ],
+        timeDimensions: [{
+          dimension: 'EmptyHourVisitors.createdAt',
+          granularity: 'hour',
+          dateRange: ['2017-01-01', '2017-01-25']
+        }],
+        timezone: 'UTC',
+        order: [{
+          id: 'EmptyHourVisitors.createdAt'
+        }],
+        preAggregationsSchema: ''
+      });
+
+      const preAggregations = cubeEvaluator.scheduledPreAggregations();
+      const partitionedPreAgg =
+        preAggregations.find(p => p.preAggregationName === 'emptyPartitioned' && p.cube === 'visitor_checkins');
+
+      const minMaxQueries = query.preAggregationStartEndQueries('visitor_checkins', partitionedPreAgg.preAggregation);
+
+      console.log(minMaxQueries);
+
+      const res = await dbRunner.testQueries(minMaxQueries);
+
+      res.should.be.deepEqual(
+        [{ max: null }]
+      );
+    });
+  });
+
   it('mutable partition default refreshKey', () => {
     return compiler.compile().then(() => {
       const query = new PostgresQuery({ joinGraph, cubeEvaluator, compiler }, {

packages/cubejs-server-core/core/RefreshScheduler.js

@@ -31,12 +31,18 @@ class RefreshScheduler {
         return data[0] && data[0][Object.keys(data[0])[0]];
       };
 
+      const dateRange = [extractDate(startDate), extractDate(endDate)];
+      if (!dateRange[0] || !dateRange[1]) {
+        // Empty table. Nothing to refresh.
+        return [];
+      }
+
       const baseQuery = {
         ...queryingOptions,
         ...preAggregation.references,
         timeDimensions: [{
           ...preAggregation.references.timeDimensions[0],
-          dateRange: [extractDate(startDate), extractDate(endDate)]
+          dateRange
         }]
       };
       const partitionQuery = compilerApi.createQuery(compilers, dbType, baseQuery);