nickva commented on issue #3517:
URL: https://github.com/apache/couchdb/issues/3517#issuecomment-891897388
Discussing this on couchdb-dev channel, CouchDB with JS runtimes later than
mozjs 60 need to re-write their design doc functions [1]. Also, unlike pure map
views, where the JS runtime gets to process (learn) the function first, then
only handle the keys and values [2], custom reduce functions have the function
body sent with each query server command. Therefore, any overhead in that JS
code re-writing is amplified for the custom reducers.
Looking back at the reduce function, I wonder if it would be possible to
rewrite it using two _sum[3] and _stats[4] built-in reducers, which should not
suffer from the same rewriting overhead.
For example, the _sum reducer can be applied to objects, such as:
```
"views":{"v1":{ "map": "function(doc){ emit(null, {x:doc.x, y:doc.y}) };",
"reduce":"_sum"}}
...
{"x": 1, "y": 3}
{"x": 2, "y": 4}
```
And would emit:
```
{"key":null,"value":{"x":3,"y":7}}
```
That could be used for "unsuccessful" results. For the "successful" case,
execution stats could be accumulated using _stats [3]
[1]
https://github.com/apache/couchdb/blob/0059b8f90e58e10b199a4b768a06a762d12a30d3/share/server/60/rewrite_fun.js#L29-L38
[2]
https://docs.couchdb.org/en/stable/query-server/protocol.html?highlight=add_fun#add-fun
[3] https://docs.couchdb.org/en/stable/ddocs/ddocs.html#_sum
[4] https://docs.couchdb.org/en/stable/ddocs/ddocs.html#_stats
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