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" Current storage of cells in a row is a union of vector and set. The vector holds 5 cell_and_hash's inline, up to 32 ones in the external storage and then it's switched to std::set. Once switched, the whole union becomes the waste of space, as it's size is sizeof(vector head) + 5 * sizeof(cell and hash) = 90+ bytes and only 3 pointers from it are used (std::set header). Also the overhead to keep cell_and_hash as a set entry is more then the size of the structure itself. Column ids are 32-bit integers that most likely come sequentialy. For this kind of a search key a radix tree (with some care for non-sequential cases) can be beneficial. This set introduces a compact radix tree, that uses 7-bit sub values from the search key to index on each node and compacts the nodes themselves for better memory usage. Then the row::_storage is replaced with the new tree. The most notable result is the memory footprint decrease, for wide rows down to 2x times. The performance of micro-benchmarks is a bit lower for small rows and (!) higer for longer (8+ cells). The numbers are in patch #12 (spoiler: they are better than for v2) v3: - trimmed size of radix down to 7 bits - simplified the nodes layouts, now there are 2 of them (was 4) - enhanced perf_mutation to test N-cells schema - added AVX intra-nodes search for medium-sized nodes - added .clone_from() method that helped to improve perf_mutation - minor - changed functions not to return values via refs-arguments - fixed nested classes to properly use language constructors - renamed index_to to key_t to distinguish from node_index_t - improved recurring variadic templates not to use sentinel argument - use standard concepts v2: - fixed potential mis-compilation due to strict-aliasing violation - added oracle test (radix tree is compared with std::map) - added radix to perf_collection - cosmetic changes (concepts, comments, names) A note on item 1 from v2 changelog. The nodes are no longer packed perfectly, each has grown 3 bytes. But it turned out that when used as cells container most of this growth drowned in lsa alignments. next todo: - aarch64 version of 16-keys node search tests: unit(dev), unit(debug for radix*), pref(dev) " * 'br-radix-tree-for-cells-3' of https://github.com/xemul/scylla: test/memory_footpring: Print radix tree node sizes row: Remove old storages row: Prepare row::equal for switch row: Prepare row::difference for switch row: Introduce radix tree storage type row-equal: Re-declare the cells_equal lambda test: Add tests for radix tree utils: Compact radix tree array-search: Add helpers to search for a byte in array test/perf_collection: Add callback to check the speed of clone test/perf_mutation: Add option to run with more than 1 columns test/perf_mutation: Prepare to have several regular columns test/perf_mutation: Use builder to build schema