2023-09-13 16:50:44 +00:00
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// Copyright (c) Meta Platforms, Inc. and affiliates.
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Introduce a wide column aware MergeOperator API (#11807)
Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11807
For now, RocksDB has limited support for using merge with wide columns: when a bunch of merge operands have to be applied to a wide-column base value, RocksDB currently passes only the value of the default column to the application's `MergeOperator`, which means there is no way to update any other columns during a merge. As a first step in making this more general, the patch adds a new API `FullMergeV3` to `MergeOperator`.
`FullMergeV3`'s interface enables applications to receive a plain, wide-column, or non-existent base value as merge input, and to produce a new plain value, a new wide-column value, or an existing operand as merge output. Note that there are no limitations on the column names and values if the merge result is a wide-column entity. Also, the interface is general in the sense that it makes it possible e.g. for a merge that takes a plain base value and some deltas to produce a wide-column entity as a result.
For backward compatibility, the default implementation of `FullMergeV3` falls back to `FullMergeV2` and implements the current logic where merge operands are applied to the default column of the base entity and any other columns are unchanged. (Note that with `FullMergeV3` in the `MergeOperator` interface, this behavior will become customizable.)
This patch just introduces the new API and the default backward compatible implementation. I plan to integrate `FullMergeV3` into the query and compaction logic in subsequent diffs.
Reviewed By: jaykorean
Differential Revision: D49117253
fbshipit-source-id: 109e016f25cd130fc504790818d927bae7fec6bd
2023-09-11 19:13:58 +00:00
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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// A helper template that can combine multiple functors into a single one to be
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// used with std::visit for example. It also works with lambdas, since it
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// comes with an explicit deduction guide.
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template <typename... Ts>
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struct overload : Ts... {
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using Ts::operator()...;
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};
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template <typename... Ts>
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overload(Ts...) -> overload<Ts...>;
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} // namespace ROCKSDB_NAMESPACE
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