initial sync of "limit to last N" query taking much longer when the array size gets larger?

[James Manning]

I have a firebase array that a node process appends to and browser clients query with limitToLast(20).  What I noticed was that as the array grew significantly larger, the amount of time it took the client to get the initial 20 items took longer and longer.  It felt linear, almost like the server side is walking through the array from the front to figure out the last 20 items.  Once the initial sync is done, the speed of client updates appears to be the same regardless of the array size.

Is this intentional / By Design?  Is it a result of firebase not having 'real' array support?  Is there anything I can do (outside of keeping the array smaller) to speed up the initial sync?

Thanks, everyone!!

James

[Jonny Dimond]

Hey James,

this might be due to a missing index. If you call orderByChild/orderByValue, you need to add an index to your Firebase rules. If you don't do that, limitToLast will indeed be very slow and inefficient. Let us know if you have already added an index to see if there is a different underlying issue. Thanks.

Jonny

[Emil Anticevic]

Jonny - so if you use orderByKey().limitToLast(10) the linear slowdown should NOT be the case?

[Jonny Dimond]

In general orderByKey().limitToLast(10) should not see a linear slowdown. Keep in mind that there might be caching effects here and for an app that is not used often the first listen of *any* client might involve a slowdown compared to subsequent listens...

[Emil Anticevic]

Ah ok. Is the caching done per "tree" or per app?

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[Jonny Dimond]

The caching is an underlying implementation detail so I won't go into details here. If you think that your queries are slower than expected, please ping us at sup...@firebase.com and we can investigate further. Thanks.