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Bug fix for pushing a projection down to a table in a subquery more than once #282

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merged 2 commits into from
Feb 2, 2021
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Bug fix for pushing a projection down to a table in a subquery more than once #282

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7e8ccfe
Bug fix for pushing a projection down to a table in a subquery more t…
zachmu Feb 2, 2021
3c218d7
Added even more hacky logic to projection pushdown check
zachmu Feb 2, 2021
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16 changes: 15 additions & 1 deletion sql/analyzer/pushdown.go
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Original file line number Diff line number Diff line change
Expand Up @@ -93,7 +93,21 @@ func canProject(n sql.Node, a *Analyzer) bool {
return false
}

return true
// Because analysis runs more than once on subquery, it's possible for projection pushdown logic to be applied
// multiple times. It's totally undefined what happens when you push a projection down to a table that already has
// one, and shouldn't happen. We don't have the necessary interface to interrogate a projected table about its
// projection, so we do this for now.
// TODO: this is a hack, we shouldn't use decorator nodes for logic like this.
alreadyPushedDown := false
plan.Inspect(n, func(n sql.Node) bool {
if n, ok := n.(*plan.DecoratedNode); ok && strings.Contains(n.String(), "Projected table access on") {
alreadyPushedDown = true
return false
}
return true
})

return !alreadyPushedDown
}

// canDoPushdown returns whether the node given can safely be analyzed for pushdown
Expand Down