Performance issue querying a view.

[enricogior]

@czyzm reported the issue:

There are 4 tests:
1.      PouchDB used in node on desktop (https://github.com/czyzm/TestViewsNode).
2.      PouchDB used in jxcore on desktop (https://github.com/czyzm/TestViewsJx).
3.      PouchDB used in WebView on mobile (https://github.com/czyzm/TestViewsMobile).
4.      PouchDB used in jxcore on mobile (https://github.com/czyzm/TestViewsMobile).
 
All tests have the same scenario:
-          Create DB
-          Insert 2000 documents into DB
-          Insert design doc (view) into DB
-          Run the query and calculate the time of its execution 
           (the first query launches view build)
 
Here are the results:
Test                 Query exec time (2000 docs) Query exec time (500 docs)
1. Node (desktop)    2.5s                        0.5s
2. Jxcore (desktop)  3s                          0.5s
3. WebView (S7)      11s                         3s
4. Jxcore (S7)       90s                         21s
 

https://github.com/czyzm/TestViewsMobile
https://github.com/czyzm/TestViewsNode
https://github.com/czyzm/TestViewsJx

[enricogior]

The execution time is linear with the number of documents inserted in the DB:

Docs #    Exec time
 100       4s
 500      19s
1000      43s
2000      85s

[enricogior]

Insert execution time doesn't show performance issues, on the contrary, in the WebView it takes 29s while using JXcore only 13s.

[yaronyg]

@enricogior The question going through my head is - is this a failure in PouchDB or Leveldown-Mobile? The evidence argues against PouchDB (since the Webview perf is reasonable and the same code should be used in both places) but a quick way to check is to switch the test from using leveldown to using the in memory adapter.

See https://pouchdb.com/adapters.html#pouchdb_in_node_js for details but all you should have to do is run 'npm install memdown' in the project and then replace https://github.com/czyzm/TestViewsMobile/blob/211771e1edca9d4db49099135114b20a884e5b29/www/jxcore/app.js#L41 with 'db: require('memdown'),' and then re-run the test.

If you see the same bad perf then it's a PouchDB bug and you should bounce it back to me. If you see reasonable perf then that is pretty good evidence that something is wrong with leveldown-mobile.

[enricogior]

These are the test results using MemDown instead of LevelDown-Mobile:

Docs #    Exec time
 100       6s
 500      25s
1000      53s
2000      114s

[enricogior]

@yaronyg we can close this issue in the leveldown-mobile repo and open a new one specific for PouchDB.

[yaronyg]

Please test with spidermonkey with JIT on desktop just to see what our perf looks like. Also we should try memdown on desktop with normal node and normal jxcore (v8).