# Matrix multiplication of a view of QR.Q with Vector{VariableRef} is slow

**URL:** <https://discourse.julialang.org/t/matrix-multiplication-of-a-view-of-qr-q-with-vector-variableref-is-slow/74480>\
**Category:** Performance\
**Tags:** jump, linearalgebra, sparse\
**Created:** [January 12, 2022, 12:11pm UTC](https://discourse.julialang.org/t/matrix-multiplication-of-a-view-of-qr-q-with-vector-variableref-is-slow/74480 "2022-01-12T12:11:48Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [January 12, 2022, 12:48pm UTC](https://discourse.julialang.org/t/matrix-multiplication-of-a-view-of-qr-q-with-vector-variableref-is-slow/74480/2 "2022-01-12T12:48:40Z")

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See: [What is the most efficient way of obtaining the orthogonal column space of a matrix? - #13 by stevengj](https://discourse.julialang.org/t/what-is-the-most-efficient-way-of-obtaining-the-orthogonal-column-space-of-a-matrix/72212/13)

We should really have an efficient `getindex` method for slicing `Q`, but no one has implemented one yet. We could similarly implement a more efficient method for working with views of `Q`.

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_[View the full topic](https://discourse.julialang.org/t/matrix-multiplication-of-a-view-of-qr-q-with-vector-variableref-is-slow/74480)._