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This code isn't used, and there's no clear reason why anybody would ever want to use it. These traversal mechanisms don't yield a visitation order that is semantically meaningful for any external purpose, nor are they any faster or simpler than the left-to-right or right-to-left traversals. (In fact, some rough testing suggests they are slower :-(.) Moreover, these mechanisms are impossible to test in any arm's-length fashion; doing so requires knowledge of the red-black tree's internal implementation. Hence, let's just jettison them. Discussion: https://postgr.es/m/17735.1505003111@sss.pgh.pa.us
This directory contains a general purpose data structures, for use anywhere in the backend: binaryheap.c - a binary heap hyperloglog.c - a streaming cardinality estimator pairingheap.c - a pairing heap rbtree.c - a red-black tree ilist.c - single and double-linked lists. stringinfo.c - an extensible string type Aside from the inherent characteristics of the data structures, there are a few practical differences between the binary heap and the pairing heap. The binary heap is fully allocated at creation, and cannot be expanded beyond the allocated size. The pairing heap on the other hand has no inherent maximum size, but the caller needs to allocate each element being stored in the heap, while the binary heap works with plain Datums or pointers. The linked-lists in ilist.c can be embedded directly into other structs, as opposed to the List interface in nodes/pg_list.h.