docs/victorialogs/logsql.md: document how to filter out empty field values when calculating min, max, median and quantile

The min, max, median and quantile functions work with string field values, since strings can be sorted.
So there is min, max, median and quantile across string values.

Empty field fields equal to non-existing values according to VictoriaLogs data model
(see https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model ).
If some log has missing field requested in the min, max, median or quantile functions,
then these functions receive an empty value. Such an empty value can affect function results
in unexpected ways. So it is recommended removing empty results if needed
with the conditional stats functionality ( https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters ):

   min(some_field) if (some_field:*)

The `some_field:*` filter skips empty values ( see https://docs.victoriametrics.com/victorialogs/logsql/#any-value-filter )

Updates #95

Author: valyala

docs/victorialogs/logsql.md

@@ -4532,6 +4532,11 @@ over logs for the last 5 minutes:
 _time:5m | stats max(duration) max_duration
 ```
 
+The `max(some_field)` function works with string values for the `some_field`, so it returns an empty string value if `some_field`
+is missing in some of the processed logs according to [VictoriaLogs data model](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model).
+Use `max(some_field) if (some_field:*) as min_value_without_empty_string` syntax for filtering out empty string values.
+See [conditional stats docs](https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters) for more details.
+
 It is possible to calculate the maximum value across all the fields with common prefix via `max(prefix*)` syntax.
 
 [`row_max`](https://docs.victoriametrics.com/victorialogs/logsql/#row_max-stats) function can be used for obtaining other fields with the maximum duration.
@@ -4555,6 +4560,11 @@ over logs for the last 5 minutes:
 _time:5m | stats median(duration) median_duration
 ```
 
+The `median(some_field)` function works with string values for the `some_field`, so it returns an empty string value if `some_field`
+is missing in some of the processed logs according to [VictoriaLogs data model](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model).
+Use `median(some_field) if (some_field:*) as min_value_without_empty_string` syntax for filtering out empty string values.
+See [conditional stats docs](https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters) for more details.
+
 It is possible to calculate the median across all the fields with common prefix via `median(prefix*)` syntax.
 
 See also:
@@ -4574,6 +4584,11 @@ over logs for the last 5 minutes:
 _time:5m | stats min(duration) min_duration
 ```
 
+The `min(some_field)` function works with string values for the `some_field`, so it returns an empty string value if `some_field`
+is missing in some of the processed logs according to [VictoriaLogs data model](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model).
+Use `min(some_field) if (some_field:*) as min_value_without_empty_string` syntax for filtering out empty string values.
+See [conditional stats docs](https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters) for more details.
+
 It is possible to find the minimum across all the fields with common prefix via `min(prefix*)` syntax.
 
 [`row_min`](https://docs.victoriametrics.com/victorialogs/logsql/#row_min-stats) function can be used for obtaining other fields with the minimum duration.
@@ -4601,6 +4616,11 @@ _time:5m | stats
   quantile(0.99, request_duration_seconds) p99
 ```
 
+The `quantile(phi, some_field)` function works with string values for the `some_field`, so it returns an empty string value if `some_field`
+is missing in some of the processed logs according to [VictoriaLogs data model](https://docs.victoriametrics.com/victorialogs/keyconcepts/#data-model).
+Use `quantile(phi, some_field) if (some_field:*) as min_value_without_empty_string` syntax for filtering out empty string values.
+See [conditional stats docs](https://docs.victoriametrics.com/victorialogs/logsql/#stats-with-additional-filters) for more details.
+
 It is possible to calculate the quantile across all the fields with common prefix via `quantile(phi, prefix*)` syntax.
 
 See also: