mean:
```
softwareheritage=> select avg(length) from content_language cl inner join content c on cl.id=c.sha1;
avg
--------------------
26862.385193867011
(1 row)
```
variance:
```
softwareheritage=> select variance(length) from content_language cl inner join content c on cl.id=c.sha1;
variance
-----------------------
125835685708.88915180
(1 row)
```
standard deviation:
```
softwareheritage=> select stddev(length) from content_language cl inner join content c on cl.id=c.sha1;
stddev
-----------------
355008.08759433
(1 row)
```