release-20.2: colbuilder: disable wrapping of changefeed processors #55753
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yuzefovich
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The root of the problem is that Columnarizer has buffering behavior - in 20.1 it will be hanging until coldata.BatchSize() (1024 by default) rows are emitted by the changefeed. On 20.2 and master due to dynamic batch size behavior it will still be hanging but in a slightly different manner. This is less of a problem on 20.2 and the current master because the vectorized engine will not be used for the changefeed DistSQL flow since the vectorized row count threshold is never met for it (the estimated row count for the plan is 0, so unless a user does `SET vectorize_row_count_threshold=0;` or `SET vectorize=experimental_always;`, we will always use row-by-row engine). In 20.1 the meaning of `vectorize=on` was different - we never looked at the threshold and used the vectorized engine if it was supported. In order to fix this issue we simply refuse to wrap the changefeed processors, so the row-by-row engine will be always used for changefeed flows. Release note (bug fix): The current implementation of changefeeds is incompatible with the vectorized engine, so whenever the latter is being used to run the former, it could hang indefinitely. This is now fixed. Namely, on 20.2 releases this could happen if the user runs `SET vectorize_row_count_threshold=0;`, and on 20.1 releases - if the user runs `SET vectorize=on`.
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Backport 1/1 commits from #55616.
/cc @cockroachdb/release
The root of the problem is that
Columnarizerhas buffering behavior -in 20.1 it will be hanging until
coldata.BatchSize()(1024 by default)rows are emitted by the changefeed. On 20.2 and master due to dynamic
batch size behavior it will still be hanging but in a slightly
different manner.
This is less of a problem on 20.2 and the current master because the
vectorized engine will not be used for the changefeed DistSQL flow since
the vectorized row count threshold is never met for it (the estimated
row count for the plan is 0, so unless a user does
SET vectorize_row_count_threshold=0;orSET vectorize=experimental_always;, we will always use row-by-rowengine). In 20.1 the meaning of
vectorize=onwas different - we neverlooked at the threshold and used the vectorized engine if it was
supported.
In order to fix this issue we simply refuse to wrap the changefeed
processors, so the row-by-row engine will be always used for changefeed
flows.
Fixes: #55605.
Release note (bug fix): The current implementation of changefeeds is
incompatible with the vectorized engine, so whenever the latter is being
used to run the former, it could hang indefinitely. This is now fixed.
Namely, on 20.2 releases this could happen if the user runs
SET vectorize_row_count_threshold=0;, and on 20.1 releases - if theuser runs
SET vectorize=on.