actually drop old cache metrics via timestamp.

frmcache/src/app/cache_worker.go

@@ -138,19 +138,11 @@ func (c *CacheWorker) cacheMetricsWithHistory(metric string, data []string) (err
 		}
 	}
 
-	//720 = 1 hour, 5 second increments. retain that many rows for every data.
-	keep := 720 * len(data)
-
 	delete := `delete from cache_with_history where
 metric = $1 AND
 url = $2 AND session_name = $3 AND
-id NOT IN (
-select id from "cache_with_history" where metric = $1
-AND url = $2 AND session_name = $3
-order by id desc
-limit $4
-);`
-	_, err = tx.Exec(delete, metric, c.frmBaseUrl, c.sessionName, keep)
+time < $4;`
+	_, err = tx.Exec(delete, metric, c.frmBaseUrl, c.sessionName, c.now.Add(time.Duration(-1) * time.Hour))
 	return
 }
 

frmcache/src/app/cache_worker_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 )
 
 func setupServer(expectedResponse string) *httptest.Server {
@@ -51,7 +52,7 @@ func TestCacheHistory(t *testing.T) {
 	mock.ExpectBegin()
 	mock.ExpectExec(`^insert into cache_with_history (.+)`).WithArgs("drone", `{"ID":"0","VehicleType":"Drone"}`, now, frm.URL, sessionName).WillReturnResult(sql.NewResult(1, 1))
 	mock.ExpectExec(`^insert into cache_with_history (.+)`).WithArgs("drone", `{"ID":"1","VehicleType":"Drone"}`, now, frm.URL, sessionName).WillReturnResult(sql.NewResult(1, 1))
-	mock.ExpectExec(`^delete from cache_with_history (.+)`).WithArgs("drone", frm.URL, sessionName, 720*2).WillReturnResult(sql.NewResult(1, 1))
+	mock.ExpectExec(`^delete from cache_with_history (.+)`).WithArgs("drone", frm.URL, sessionName, now.Add(time.Duration(-1)*time.Hour)).WillReturnResult(sql.NewResult(1, 1))
 	mock.ExpectCommit()
 
 	c := NewCacheWorker(frm.URL, db)

frmcache/src/db/007_add_cache_history_metric_index.sql

@@ -0,0 +1,12 @@
+-- Write your migrate up statements here
+
+CREATE INDEX cache_with_history_metric ON cache_with_history(metric);
+CREATE INDEX cache_with_history_time ON cache_with_history(time);
+
+---- create above / drop below ----
+
+-- Write your migrate down statements here. If this migration is irreversible
+-- Then delete the separator line above.
+
+DROP INDEX if exists cache_with_history_metric;
+DROP INDEX if exists cache_with_history_time;