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MLIR: document the rows a bare constant column stands for #1182

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MLIR: document the rows a bare constant column stands for #1182

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6 changes: 6 additions & 0 deletions query/ir/dialect/db/DBOps.td
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Expand Up @@ -2909,6 +2909,12 @@ def BroadcastConstant : TuringOp<"broadcast_constant", [Pure]> {
after the barrier - a WHERE cutting those rows, a count(*) tallying them, a pattern
crossed with them - read a column that has them.

Elsewhere a bare constant needs no broadcast. An op reading constants alone, such as
the db.output of `MATCH (n) RETURN 5`, runs over the rows in flight where it stands,
and lowering takes those rows from the innermost loop. They are not a use of any
column, so an analysis over use-def chains must not read an op with no user as one
whose rows nothing reads: fuse_explore_distinct_ends refuses such an exploration.

The driver is absent when no relation drives the scope, which is then the single row
the constants are - `WITH 1 AS x WHERE x > 100`, whose one row the filter drops - so
the value is laid out over that row:
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