Fix to consolidate avg accuracy (#753)

* The interval must fit exactly n times to the time range and query made so that the resulting avg will be exact

* Add interval fix also to trend query

src/datasource-zabbix/zabbix/connectors/sql/mysql.js

@@ -3,26 +3,24 @@
  */
 
 function historyQuery(itemids, table, timeFrom, timeTill, intervalSec, aggFunction) {
-  let time_expression = `clock DIV ${intervalSec} * ${intervalSec}`;
   let query = `
-    SELECT CAST(itemid AS CHAR) AS metric, ${time_expression} AS time_sec, ${aggFunction}(value) AS value
+    SELECT CAST(itemid AS CHAR) AS metric, MIN(clock) AS time_sec, ${aggFunction}(value) AS value
     FROM ${table}
     WHERE itemid IN (${itemids})
       AND clock > ${timeFrom} AND clock < ${timeTill}
-    GROUP BY ${time_expression}, metric
+    GROUP BY (clock-${timeFrom}) DIV ${intervalSec}, metric
     ORDER BY time_sec ASC
   `;
   return query;
 }
 
 function trendsQuery(itemids, table, timeFrom, timeTill, intervalSec, aggFunction, valueColumn) {
-  let time_expression = `clock DIV ${intervalSec} * ${intervalSec}`;
   let query = `
-    SELECT CAST(itemid AS CHAR) AS metric, ${time_expression} AS time_sec, ${aggFunction}(${valueColumn}) AS value
+    SELECT CAST(itemid AS CHAR) AS metric, MIN(clock) AS time_sec, ${aggFunction}(${valueColumn}) AS value
     FROM ${table}
     WHERE itemid IN (${itemids})
       AND clock > ${timeFrom} AND clock < ${timeTill}
-    GROUP BY ${time_expression}, metric
+    GROUP BY (clock-${timeFrom}) DIV ${intervalSec}, metric
     ORDER BY time_sec ASC
   `;
   return query;

src/datasource-zabbix/zabbix/connectors/sql/sqlConnector.js

@@ -43,6 +43,12 @@ export class SQLConnector extends DBConnector {
     let {intervalMs, consolidateBy} = options;
     let intervalSec = Math.ceil(intervalMs / 1000);
 
+    // The interval must match the time range exactly n times, otherwise
+    // the resulting first and last data points will yield invalid values in the
+    // calculated average value in downsampleSeries - when using consolidateBy(avg)
+    let numOfIntervals = Math.ceil((timeTill - timeFrom) / intervalSec);
+    intervalSec = (timeTill - timeFrom) / numOfIntervals;
+
     consolidateBy = consolidateBy || 'avg';
     let aggFunction = dbConnector.consolidateByFunc[consolidateBy];
 
@@ -66,6 +72,12 @@ export class SQLConnector extends DBConnector {
     let { intervalMs, consolidateBy } = options;
     let intervalSec = Math.ceil(intervalMs / 1000);
 
+    // The interval must match the time range exactly n times, otherwise
+    // the resulting first and last data points will yield invalid values in the
+    // calculated average value in downsampleSeries - when using consolidateBy(avg)
+    let numOfIntervals = Math.ceil((timeTill - timeFrom) / intervalSec);
+    intervalSec = (timeTill - timeFrom) / numOfIntervals;
+
     consolidateBy = consolidateBy || 'avg';
     let aggFunction = dbConnector.consolidateByFunc[consolidateBy];