docs: update db connection docs

docs/sources/img/installation-postgres_ds_config.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:d7dae33170bf09b353f886bc1a8cbc4307d80858c514199f08f1b9e01bc01e81
+size 260406

docs/sources/installation/configuration-sql.md

@@ -1,5 +1,7 @@
 # SQL Data Source Configuration
 
+## MySQL
+
 In order to use _Direct DB Connection_ feature you should configure SQL data source first.
 
 ![Configure MySQL data source](../img/installation-mysql_ds_config.png)
@@ -7,7 +9,7 @@ In order to use _Direct DB Connection_ feature you should configure SQL data sou
 Select _MySQL_ data source type and provide your database host address and port (3306 is default for MySQL). Fill
 database name (usually, `zabbix`) and specify credentials.
 
-## Security notes
+### Security notes
 
 As you can see in _User Permission_ note, Grafana doesn't restrict any queries to the database. So you should be careful
 and create a special user with limited access to Zabbix database. Grafana-Zabbix plugin uses only `SELECT` queries to
@@ -20,3 +22,13 @@ Also, all queries are invoked by grafana-server, so you can restrict connection
 ```sql
 GRANT SELECT ON zabbix.* TO 'grafana'@'grafana-host' identified by 'password';
 ```
+
+## PostgreSQL
+
+Select _PostgreSQL_ data source type and provide your database host address and port (5432 is default). Fill
+database name (usually, `zabbix`) and specify credentials.
+
+![Configure PostgreSQL data source](../img/installation-postgres_ds_config.png)
+### Security notes
+
+Make sure you use read-only user for Zabbix database.

docs/sources/installation/configuration.md

@@ -65,7 +65,7 @@ Read [how to configure](/installation/configuration-sql) SQL data source in Graf
 
 #### Supported databases
 
-Now only **MySQL** is supported by Grafana.
+**MySQL** and **PostgreSQL** are supported by Grafana.
 
 ### Alerting
 

docs/sources/reference/direct-db-connection.md

@@ -22,12 +22,26 @@ and `trend.get` API calls.
 
 Below is an example query for getting history in the Grafana-Zabbix Plugin:
 
+**MySQL**:
 ```sql
 SELECT itemid AS metric, clock AS time_sec, {aggFunc}(value) as value
 FROM {historyTable}
 WHERE itemid IN ({itemids})
   AND clock > {timeFrom} AND clock < {timeTill}
 GROUP BY time_sec DIV {intervalSec}, metric
+ORDER BY time_sec ASC
+```
+
+**PostgreSQL**:
+```sql
+SELECT to_char(itemid, 'FM99999999999999999999') AS metric, 
+  clock / {intervalSec} * {intervalSec} AS time, 
+  {aggFunc}(value) AS value
+FROM {historyTable}
+WHERE itemid IN ({itemids})
+  AND clock > {timeFrom} AND clock < {timeTill}
+GROUP BY 1, 2
+ORDER BY time ASC
 ```
 
 where `{aggFunc}` is one of `[AVG, MIN, MAX, SUM, COUNT]` aggregation functions, `{historyTable}` is a history table,
@@ -36,14 +50,30 @@ where `{aggFunc}` is one of `[AVG, MIN, MAX, SUM, COUNT]` aggregation functions,
 When getting trends, the plugin additionally queries a particular value column (`value_avg`, `value_min` or `value_max`) which
 depends on `consolidateBy` function value:
 
+**MySQL**:
 ```sql
 SELECT itemid AS metric, clock AS time_sec, {aggFunc}({valueColumn}) as value
 FROM {trendsTable}
 WHERE itemid IN ({itemids})
   AND clock > {timeFrom} AND clock < {timeTill}
 GROUP BY time_sec DIV {intervalSec}, metric
+ORDER BY time_sec ASC
 ```
 
+**PostgreSQL**:
+```sql
+SELECT to_char(itemid, 'FM99999999999999999999') AS metric, 
+  clock / {intervalSec} * {intervalSec} AS time, 
+  {aggFunc}({valueColumn}) AS value
+FROM {trendsTable}
+WHERE itemid IN ({itemids})
+  AND clock > {timeFrom} AND clock < {timeTill}
+GROUP BY 1, 2
+ORDER BY time ASC
+```
+
+**Note**: these queries may be changed in future, so look into sources for actual query structure.
+
 As you can see, the Grafana-Zabbix plugin uses aggregation by a given time interval. This interval is provided by Grafana and depends on the panel width in pixels. Thus, Grafana displays the data in the proper resolution.
 
 ## Functions usage with Direct DB Connection