From dev-return-49204-archive-asf-public=cust-asf.ponee.io@couchdb.apache.org Thu Mar 26 11:16:20 2020 Return-Path: X-Original-To: archive-asf-public@cust-asf.ponee.io Delivered-To: archive-asf-public@cust-asf.ponee.io Received: from mail.apache.org (hermes.apache.org [207.244.88.153]) by mx-eu-01.ponee.io (Postfix) with SMTP id 36C5E180637 for ; Thu, 26 Mar 2020 12:16:20 +0100 (CET) Received: (qmail 777 invoked by uid 500); 26 Mar 2020 11:16:19 -0000 Mailing-List: contact dev-help@couchdb.apache.org; run by ezmlm Precedence: bulk List-Help: List-Unsubscribe: List-Post: List-Id: Reply-To: dev@couchdb.apache.org Delivered-To: mailing list dev@couchdb.apache.org Received: (qmail 764 invoked by uid 99); 26 Mar 2020 11:16:19 -0000 Received: from Unknown (HELO mailrelay1-lw-us.apache.org) (10.10.3.159) by apache.org (qpsmtpd/0.29) with ESMTP; Thu, 26 Mar 2020 11:16:19 +0000 Received: from mail-lj1-f175.google.com (mail-lj1-f175.google.com [209.85.208.175]) by mailrelay1-lw-us.apache.org (ASF Mail Server at mailrelay1-lw-us.apache.org) with ESMTPSA id E145BF8C for ; Thu, 26 Mar 2020 11:16:18 +0000 (UTC) Received: by mail-lj1-f175.google.com with SMTP id t17so5927218ljc.12 for ; Thu, 26 Mar 2020 04:16:18 -0700 (PDT) X-Gm-Message-State: AGi0PubKD8liS2W5z8NN3dq1weHy+tFG2T0Hjfn8X07t65EOHv4hJBv7 CPbPlV7B+bfrk2Q9O/WgRAz0onqgDNIHeeNf4ws+jg== X-Google-Smtp-Source: APiQypICayuc438xF4BRmHv2dCIvAdXHwAK+9fhXksxLWI0lQLvtQEGpg13MnRifKu1MJAaHS3OhS3MXIAGAml6mfqE= X-Received: by 2002:a2e:988c:: with SMTP id b12mr5002953ljj.138.1585221377941; Thu, 26 Mar 2020 04:16:17 -0700 (PDT) MIME-Version: 1.0 References: <4ebfca4c-5537-49d6-8e2c-6ea4a98ca69b@www.fastmail.com> <83da7901-6baa-f153-1bff-d60877eee403@apache.org> <518C26B2-0CD2-4FDA-84D1-EDD346A2DBFA@apache.org> In-Reply-To: From: Garren Smith Date: Thu, 26 Mar 2020 13:16:06 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [DISCUSS] Mango indexes on FDB To: CouchDB Developers Content-Type: multipart/alternative; boundary="0000000000001f72ef05a1c01db0" --0000000000001f72ef05a1c01db0 Content-Type: text/plain; charset="UTF-8" On Thu, Mar 26, 2020 at 12:46 PM Glynn Bird wrote: > +1 on removing step 3 - my reservation on falling back on all_docs is that > users have no insight into how expensive a query is, other than measuring > latencies (which might depend on other factors). I would hope that folks > would use option 1 anyway. > > So Paul's list becomes: > > 1. If user specifies an index, use it even if we have to wait > 2. If an index is built that can be used, use it > 3. n/a > 4. As a last resort use _all_docs > +1 > > > On Thu, 26 Mar 2020 at 10:33, Will Holley wrote: > > > Ah - in that case I think we should remove step 3, as it leads to a > > confusing mental model. It's much simpler to explain that Mango will only > > use fresh indexes and any new indexes will build in the background. > > > > On Thu, 26 Mar 2020 at 10:15, Garren Smith wrote: > > > > > On Thu, Mar 26, 2020 at 11:04 AM Will Holley > > wrote: > > > > > > > Broadly, I think it's a big step forward if we can prevent Mango from > > > > automatically selecting extremely stale indexes. > > > > > > > > I've been going back and forth on whether step 3 could lead to some > > > > difficult-to-predict behaviour. If we assume that requests have a > short > > > > timeout - e.g. we can't return any result if it doesn't complete > within > > > the > > > > FDB transaction timeout - then I think it's fine: queries that use > > > > _all_docs and a large database will be timing out anyway. > > > > > > > > If we were to allow long-running queries then it seems a bit > sketchier > > > > because adding an index to a large database could cause queries that > > > > previously completed to start timing out whilst they block on the > index > > > > build. This is basically how Mango in CouchDB 2/3 behaves and has > been > > a > > > > big pain point for customers I've worked with, to the point where you > > > > basically need to explicitly specify which index Mango uses in all > > cases > > > if > > > > you're to avoid surprise timeouts when somebody adds a new index. > > > > > > > > As I understand it, we're not allowing queries to span FDB > transactions > > > so > > > > this latter case is not something to worry about? > > > > > > > > > We are going to allow queries to span transactions. This is already > > > implemented for views and will be for mango > > > > > > > > > > > > > > Cheers, > > > > > > > > Will > > > > > > > > On Wed, 25 Mar 2020 at 19:43, Garren Smith > wrote: > > > > > > > > > On Wed, Mar 25, 2020 at 8:35 PM Paul Davis < > > > paul.joseph.davis@gmail.com> > > > > > wrote: > > > > > > > > > > > > It was therefore felt that having an immediate "Not ready" > signal > > > for > > > > > > just _some_ calls to _find, based on the type of backing index, > > was a > > > > bad > > > > > > and confusing api. > > > > > > > > > > > > > > We also discussed _find calls where the user does not specify > an > > > > index, > > > > > > and concluded that we would be free to choose between using the > > > > _all_docs > > > > > > index (which is always up to date but rarely the best index for a > > > given > > > > > > selector) or blocking to update a better but stale index. > > > > > > > > > > > > > > Summary-ing my summarisation; > > > > > > > > > > > > > > 1) if you specify an index, we'll use it even if we have to > > update > > > > it, > > > > > > no matter how long that takes. > > > > > > > 2) if you don't specify an index, it's the dealers choice. The > > > > details > > > > > > here may change in point releases. > > > > > > > > > > > > > > > > > > > So it seems there's still a bit of confusion on what the > consensus > > is > > > > > > here. The way that I had thought this would work is that we'd do > > > > > > something like such: > > > > > > > > > > > > 1. If user specifies and index, use it even if we have to wait > > > > > > 2. If an index is built that can be used, use it > > > > > > 3. If an index is building that can be used, wait for it > > > > > > 4. As a last resort use _all_docs > > > > > > > > > > > > Discussing with Garren on the PR he's of the opinion that we > should > > > > > > skip step 3 and just go directly to using _all_docs if nothing is > > > > > > built. > > > > > > > > > > > > > > > > I just want to clarify step 3. I'm ok with using an index that > still > > > > needs > > > > > to be built as long as there is no other built index > > > > > that can service the request. > > > > > > > > > > So the big thing for me is to always prefer a built index over a > > > building > > > > > index. In the situation where there is only 1 building index versus > > all > > > > > docs I'm ok with using the building index. > > > > > > > > > > > > > > > > > > > > > > > > > > My main assumption is that most cases where a user is creating an > > > > > > index and then wanting to run a query with it are in the > > > > > > design/exploration phase of learning the feature or designing an > > > index > > > > > > to use. In that scenario if we skip waiting it seems likely that > a > > > > > > user could easily be led to believe that an index creation > "worked" > > > > > > for their selector when in reality it was just backed by > _all_docs. > > > > > > > > > > > > The other reason for preferring to wait for an index to finish > > > > > > building is that the UI for the normal case of creating indexes > is > > a > > > > > > bit awkward. Having to run a polling loop around checking the > index > > > > > > status seems suboptimal in most cases. > > > > > > > > > > > > Am I missing other cases that would benefit from not waiting and > > just > > > > > > using _all_docs? > > > > > > > > > > > > Paul > > > > > > > > > > > > > > > > > > > > > --0000000000001f72ef05a1c01db0--