Improve comments for execExpr.c's isAssignmentIndirectionExpr().
I got confused about why this function doesn't need to recursively search the expression tree for a CaseTestExpr node. After figuring that out, add a comment to save the next person some time.
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@ -2443,14 +2443,14 @@ ExecInitArrayRef(ExprEvalStep *scratch, ArrayRef *aref, PlanState *parent,
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* refassgnexpr is itself a FieldStore or ArrayRef that needs to
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* obtain and modify the previous value of the array element or slice
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* being replaced. If so, we have to extract that value from the
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* array and pass it down via the CaseTextExpr mechanism. It's safe
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* array and pass it down via the CaseTestExpr mechanism. It's safe
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* to reuse the CASE mechanism because there cannot be a CASE between
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* here and where the value would be needed, and an array assignment
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* can't be within a CASE either. (So saving and restoring
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* innermost_caseval is just paranoia, but let's do it anyway.)
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*
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* Since fetching the old element might be a nontrivial expense, do it
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* only if the argument appears to actually need it.
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* only if the argument actually needs it.
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*/
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if (isAssignmentIndirectionExpr(aref->refassgnexpr))
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{
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@ -2506,10 +2506,16 @@ ExecInitArrayRef(ExprEvalStep *scratch, ArrayRef *aref, PlanState *parent,
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/*
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* Helper for preparing ArrayRef expressions for evaluation: is expr a nested
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* FieldStore or ArrayRef that might need the old element value passed down?
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* FieldStore or ArrayRef that needs the old element value passed down?
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*
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* (We could use this in FieldStore too, but in that case passing the old
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* value is so cheap there's no need.)
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*
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* Note: it might seem that this needs to recurse, but it does not; the
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* CaseTestExpr, if any, will be directly the arg or refexpr of the top-level
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* node. Nested-assignment situations give rise to expression trees in which
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* each level of assignment has its own CaseTestExpr, and the recursive
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* structure appears within the newvals or refassgnexpr field.
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*/
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static bool
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isAssignmentIndirectionExpr(Expr *expr)
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