Add template for a proposal for a new optimization.

Author: Andi Wang

.github/ISSUE_TEMPLATE/07-optimization.yml

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+# Copyright Materialize, Inc. and contributors. All rights reserved.
+#
+# Use of this software is governed by the Business Source License
+# included in the LICENSE file at the root of this repository.
+#
+# As of the Change Date specified in that file, in accordance with
+# the Business Source License, use of this software will be governed
+# by the Apache License, Version 2.0.
+
+name: "Internal - Proposed Optimization"
+description: A proposal for a new query optimization. Contributor use only.
+labels: ["A-optimization", "needs-discussion"]
+body:
+  - type: textarea
+    attributes:
+      label: Elevator pitch
+      description: Each question should be answered comprehensively but using as
+        few sentences as possible.
+      value: |
+        1. What does the transformation do? Give a representative SQL query.
+        2. Why should we add it?
+        3. When would it be good to have?
+        4. When would it be ineffectual?
+        5. When would be bad to have?
+        6. In the worst case, how would it degrade performance?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: List real life instances where this transformation would help.
+  - type: textarea
+    attributes:
+      label: Cost Model
+      description: |
+        If both the implementation and cost model are simple, this section can
+        be skipped if the implementation section is filled in.
+
+        Each subsection is meant to be answered with formulae. Take into account
+        memory, CPU, liveness, and skew. Approximations are ok if it is not
+        possible to calculate exact numbers.
+
+        If the transformation is always good to have, you only have to fill out
+        the first subsection.
+      value: |
+        1. What is the benefit of the transformation?
+        2. What is the overhead?
+        3. When would the transformation be worthwhile? Intuitively, this should
+        be when benefit > overhead, but sometimes a benefit with regards to X
+        comes at a cost with regards to Y, and it would be worthwhile to discuss
+        when it is worthwhile to sacrifice Y to gain a benefit in X.
+  - type: textarea
+    attributes:
+      label: List any knobs that we may need to tune or expose to the user.
+  - type: textarea
+    attributes:
+      label: Proposed implementation
+      value: |
+        1. Describe the implementation.
+        2. Which queries will do better with the given implementation?
+        3. Which queries will do worse?
+        4. Break the implementation down into stages.