WIP Fix issue #7674 about UPDATE SET(..), with indirection #7675
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Queries of the form: ``` UPDATE ... SET (a, b) = (SELECT '1', '2') UPDATE ... SET (b, a) = (SELECT '2', '1') ``` Should do the same thing, but currently the order of the attributes in `SET (...)` as rewriten for pushdown is only based on the physical ordering of the attributes in the relation. This leads to several subtle problems including situation where a DROPped then reADDed attributes will change its placement in the attribute list. There are maybe more tests to add in other situation where a SET (MULTIEXPR) is possible, though UPDATE form is pretty unique as alternatives are not supported by citus: `(INSERT .. ON CONFLICT SET (a,b).....`
ruleutils in Citus is based on PostgreSQL source code, but in PostgreSQL ruleutils is not used at the planner stage. For instance, it is assumed after parser that targetList are ordered as they were read, but it's not true after rewriter, the resulting rewrite tree is then provided to planner (and citus), but the ordering of the list is not granted anymore. It's similar to others previous issues reported and still open, as well as to other bugfixes/improvment over time, the most noticable being the ProcessIndirection() which is for domain and similar. However, the implications of this bug are huge for users of `UPDATE SET (...)`: 1. if you used to order by columns order, you're maybe safe: `SET (col1, col2, col3, ...)` 2. if you used not to order by column order: `SET (col2, col1, col3, ...)` then you probably found a problem, or you have one. Note about 1. that despite appearance and your QA, you are at risk: if physical columns ordering is changed (for example after DROPping/ADDing some), the same query which use to apparently works well will silently update other columns... As it is this code is not optimized for performance, not sure it'll be needed.
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Issue reported to Data Bene support. See #7674
The fix looks good, at least it breaks nothing and the bug not visible.
I am unsure to add all regressions tests where I did, maybe their own file?
Also the PR is not yet ready regarding citus_indent, but if you can already provide some feedback.