Fix skipping compression of first to last chunk

Due to a bug in logic, we were skipping compression of the first
to last chunk which ended over an hour ago from current time. This was
causing to have one more chunk per metric uncompressed, causing
us to use more storage than necessary.

Author: antekresic

CHANGELOG.md

@@ -19,6 +19,7 @@ We use the following categories for changes:
 
 ### Fixed
 - Fix broken `promscale_packager` telemetry field for docker envs [#1077]
+- Fix compression of old chunks thus reducing storage requirements [#1081]
 
 ## [0.8.0] - 2022-01-18
 

pkg/migrations/migration_files_generated.go

@@ -2911,21 +2911,22 @@ BEGIN
     DECLARE
         chunk_schema_name name;
         chunk_table_name name;
+        chunk_range_end timestamptz;
         chunk_num INT;
     BEGIN
-        FOR chunk_schema_name, chunk_table_name, chunk_num IN
+        FOR chunk_schema_name, chunk_table_name, chunk_range_end, chunk_num IN
             SELECT
                 chunk_schema,
                 chunk_name,
+                range_end,
                 row_number() OVER (ORDER BY range_end DESC)
             FROM timescaledb_information.chunks
             WHERE hypertable_schema = 'SCHEMA_DATA'
                 AND hypertable_name = metric_table
                 AND NOT is_compressed
-                AND range_end <= compress_before
             ORDER BY range_end ASC
         LOOP
-            CONTINUE WHEN chunk_num <= 1;
+            CONTINUE WHEN chunk_num <= 1 OR chunk_range_end > compress_before;
             PERFORM SCHEMA_CATALOG.compress_chunk_for_metric(metric_table, chunk_schema_name, chunk_table_name);
             COMMIT;
         END LOOP;

pkg/migrations/sql/idempotent/base.sql

@@ -1233,6 +1233,19 @@ func TestCustomCompressionJob(t *testing.T) {
 		if chunkIsCompressed("1970-03-01 00:00:00.001+00") {
 			t.Error("third chunk compressed too soon")
 		}
+
+		// Add an chunk at current time
+		insert = fmt.Sprintf(`INSERT INTO prom_data."%s" VALUES (NOW(), 0.1, 1);`, tableName)
+		_, err = db.Exec(context.Background(), insert)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		runCompressionJob()
+		// third chunk should be compressed since its not the last chunk anymore
+		if !chunkIsCompressed("1970-03-01 00:00:00.001+00") {
+			t.Error("third chunk not compressed when it should have been")
+		}
 	})
 }