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Stop paying attention to previousResult in InMemoryCache. (#5644)
The previousResult option was originally a way to ensure referential identity of structurally equivalent cache results, before the result caching system was introduced in #3394. It worked by returning previousResult whenever it was deeply equal to the new result. The result caching system works a bit differently, and in particular never needs to do a deep comparison of results. However, there were still a few (test) cases where previousResult seemed to have a positive effect, and removing it seemed like a breaking change, so we kept it around. In the meantime, the equality check has continued to waste CPU cycles, and the behavior of previousResult has undermined other improvements, such as freezing cache results (#4514). Even worse, previousResult effectively disabled an optimization that allowed InMemoryCache#broadcastWatches to skip unchanged queries (see comments I removed if curious). This commit restores that optimization. I realized eliminating previousResult might finally be possible while working on PR #5617, which made the result caching system more precise by depending on IDs+fields rather than just IDs. This additional precision seems to have eliminated the few remaining cases where previousResult had any meaningful benefit, as evidenced by the lack of any test changes in this commit... even among the many direct tests of previousResult in src/cache/inmemory/__tests__/diffAgainstStore.ts! The removal of previousResult is definitely a breaking change (appropriate for Apollo Client 3.0), because you can still contrive cases where some never-before-seen previousResult object just happens to be deeply equal to the new result. Also, it's fair to say that this removal will strongly discourage disabling the result caching system (which is still possible for diagnostic purposes), since we rely on result caching to get the benefits that previousResult provided.
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