Returns an ST_Geometry value.
NULL arguments are not included in the aggregation. If all values in the aggregation are null, the return value is NULL.
Because the order in which the rows are processed and per-AMP results are returned for aggregation can vary from run to run, the ordering of the values in the results can vary between different instances of identical queries. Although the order of the individual values in the returned results can vary, the overall results from the query will be the same when an identical query is run on the same data.