Building sparse array from packed arrays

In version 10.1 under Windows I find that a subtle syntax difference is critical to this issue.

The form SparseArray[{a -> b}] causes unpacking while SparseArray[a -> b] does not. Further this affects the output of SparseArray itself, not only its memory consumption during creation.

On["Packing"]

a = {#, # + 1}\[Transpose] & @ Range @ 1*^6;
b = RandomReal[1, 1*^6];
c = SparseArray[a -> b];

Developer`PackedArrayQ /@ {a,b}

MaxMemoryUsed[]
ByteCount[c]

{True, True}

85279216

24000792

In a fresh session the exact same code as above adding only the brackets around a -> b

On["Packing"]

a = {#, # + 1}\[Transpose] & @ Range @ 1*^6;
b = RandomReal[1, 1*^6];
c = SparseArray[{a -> b}];    (* <-------- CRITICAL SYNTAX DIFFERENCE *)

Developer`PackedArrayQ /@ {a,b}

MaxMemoryUsed[]
ByteCount[c]

Developer`FromPackedArray::punpack: Unpacking array with dimensions {1000000,2} in call to Rule. >>

Developer`FromPackedArray::punpack: Unpacking array with dimensions {1000000} in call to Rule. >>

{True, True}

286566696

40000728

The two objects are functionally equivalent despite the size difference:

c1 = SparseArray[a -> b];
c2 = SparseArray[{a -> b}];

ByteCount /@ {c1, c2}

c1 === c2

{24000792, 40000728}

True

I surmise that specific optimizations exist in SparseArray for exactly your application, however the syntax is brittle and the optimization is not triggered for {a -> b}.