Simon Riggs wrote:
> We *must* make CREATE INDEX CONCURRENTLY work with HOT. The good news is
> I think we can without significant difficulty.

Yeah, I think CREATE INDEX CONCURRENTLY is much easier to solve. Though
I am not completely convinced that we can do that without much changes
to CREATE INDEX CONCURRENTLY logic. For example, I believe we still
need to lock out HOT-updates before we start CREATE INDEX CONCURRENTLY.
Otherwise we might end up creating two paths to the same tuple in
the new index.

Say, we have a table with two columns (int a, int b). We have an
index on 'a' and building another index on 'b'. We got a tuple
(10, 20) in the heap. In the first phase of CREATE INDEX CONCURRENTLY,
this tuple would be indexed. If the tuple is HOT-updated to (10, 30)
before the first phase ends, the updated tuple would again get
indexed in the second phase. This would lead to two paths to the
latest visible tuple from the new index.

Am I missing something in your design that stops this from
happening ?




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TIP 9: In versions below 8.0, the planner will ignore your desire to
      choose an index scan if your joining column's datatypes do not

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