docs: fill for mysql/postgres

ref #10138

docs/sources/features/datasources/mysql.md

@@ -50,6 +50,7 @@ Macro example | Description
 *$__timeFrom()* | Will be replaced by the start of the currently active time selection. For example, *FROM_UNIXTIME(1494410783)*
 *$__timeTo()* | Will be replaced by the end of the currently active time selection. For example, *FROM_UNIXTIME(1494497183)*
 *$__timeGroup(dateColumn,'5m')* | Will be replaced by an expression usable in GROUP BY clause. For example, *cast(cast(UNIX_TIMESTAMP(dateColumn)/(300) as signed)*300 as signed) as time_sec,*
+*$__timeGroup(dateColumn,'5m',0)* | Same as above but with a fill parameter so all null values will be converted to the fill value (all null values would be set to zero using this example).
 *$__unixEpochFilter(dateColumn)* | Will be replaced by a time range filter using the specified column name with times represented as unix timestamp. For example, *dateColumn > 1494410783 AND dateColumn < 1494497183*
 *$__unixEpochFrom()* | Will be replaced by the start of the currently active time selection as unix timestamp. For example, *1494410783*
 *$__unixEpochTo()* | Will be replaced by the end of the currently active time selection as unix timestamp. For example, *1494497183*
@@ -119,6 +120,19 @@ GROUP BY 1, metric_name
 ORDER BY 1
 ```
 
+Example using the fill parameter in the $__timeGroup macro to convert null values to be zero instead:
+
+```sql
+SELECT 
+  $__timeGroup(atimestamp,'24h',0) as time_sec,
+  avg(afloat) as value, 
+  avarchar as metric 
+FROM testdata.grafana_metrics 
+WHERE $__timeFilter(atimestamp)
+GROUP BY 1, avarchar 
+ORDER BY 1
+```
+
 Currently, there is no support for a dynamic group by time based on time range & panel width.
 This is something we plan to add.
 

docs/sources/features/datasources/postgres.md

@@ -49,6 +49,7 @@ Macro example | Description
 *$__timeFrom()* | Will be replaced by the start of the currently active time selection. For example, *to_timestamp(1494410783)*
 *$__timeTo()* | Will be replaced by the end of the currently active time selection. For example, *to_timestamp(1494497183)*
 *$__timeGroup(dateColumn,'5m')* | Will be replaced by an expression usable in GROUP BY clause. For example, *(extract(epoch from dateColumn)/300)::bigint*300 AS time*
+*$__timeGroup(dateColumn,'5m', 0)* | Same as above but with a fill parameter so all null values will be converted to the fill value (all null values would be set to zero using this example).
 *$__unixEpochFilter(dateColumn)* | Will be replaced by a time range filter using the specified column name with times represented as unix timestamp. For example, *dateColumn > 1494410783 AND dateColumn < 1494497183*
 *$__unixEpochFrom()* | Will be replaced by the start of the currently active time selection as unix timestamp. For example, *1494410783*
 *$__unixEpochTo()* | Will be replaced by the end of the currently active time selection as unix timestamp. For example, *1494497183*
@@ -103,6 +104,20 @@ GROUP BY time
 ORDER BY time
 ```
 
+Example using the fill parameter in the $__timeGroup macro to convert null values to be zero instead:
+
+```sql
+SELECT
+  $__timeGroup("createdAt",'5m',0),
+  sum(value) as value,
+  measurement
+FROM public.grafana_metric
+WHERE
+  $__timeFilter("createdAt")
+GROUP BY time, measurement
+ORDER BY time
+```
+
 Example with multiple columns:
 
 ```sql