Most aggregate functions support multi-stage distributed execution in which on every node we perform a local aggregation, then we route all the intermediate results to all of the nodes (according to the grouping columns), and then we perform a final aggregation that gives us the result. For some aggregate functions the decomposition into local and final stages is trivial - e.g. in order to calculate sum we need to perform local sum and then the final sum; for other functions it is a bit more involved - e.g. in order to compute avg we calculate partial sum and count locally, then we finalize sum and count at the final stage and we render the result as final_sum / final_count.

(I believe Postgres calls the multi-stage execution a partial mode.)

The decomposition of supported functions is defined here

We are currently missing the following functions:

• sqrdiff (I think it should be possible to decompose it)
• corr
• var_pop
• stddev_pop
• covar_pop
• covar_samp
• regr_intercept
• regr_r2
• regr_slope
• regr_sxx
• regr_syy
• regr_sxy
• regr_count
• regr_avgx
• regr_avgy

avg and variance are good examples of how more complicated decompositions are handled.

cc @mneverov in case you're interested

Jira issue: CRDB-3407