1fde5112b6
GetPostsSince is used when loading posts for a channel. An opportunity for optimization is that the primary SQL query is repeated twice and then a UNION is constructed for the results. ``` SELECT * FROM Posts WHERE UpdateAt > :Time AND ChannelId = :ChannelId LIMIT 1000 ``` But we can use a CTE for this which caches the results to be reused later. This leads to the main query being executed once rather than twice. And from Postgres 12 onwards, CTEs can be inlined which opens the door to further optimizations. From the docs (https://www.postgresql.org/docs/10/queries-with.html) > A useful property of WITH queries is that they are evaluated only once per execution of the parent query, even if they are referred to more than once by the parent query or sibling WITH queries. Thus, expensive calculations that are needed in multiple places can be placed within a WITH query to avoid redundant work. Another possible application is to prevent unwanted multiple evaluations of functions with side-effects. However, the other side of this coin is that the optimizer is less able to push restrictions from the parent query down into a WITH query than an ordinary subquery. In our case, the caveat does not apply because we are only filtering columns and not rows, so we can safely use it. Following are the query plan comparisons: Old: http://tatiyants.com/pev/#/plans/plan_1599394993105 New: http://tatiyants.com/pev/#/plans/plan_1599395970886 As we can see, in old bitmap index scan+heap scan happens twice, but in the new one, it happens only once. This has been load tested with a large dataset and confirmed to exhibit good improvements.
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