fork of https://github.com/oxigraph/rocksdb and https://github.com/facebook/rocksdb for nextgraph and oxigraph
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103 lines
4.4 KiB
103 lines
4.4 KiB
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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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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// Copyright (c) 2012 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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//
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// Class for specifying user-defined functions which perform a
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// transformation on a slice. It is not required that every slice
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// belong to the domain and/or range of a function. Subclasses should
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// define InDomain and InRange to determine which slices are in either
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// of these sets respectively.
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#pragma once
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#include <string>
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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class Slice;
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/*
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* A SliceTransform is a generic pluggable way of transforming one string
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* to another. Its primary use-case is in configuring rocksdb
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* to store prefix blooms by setting prefix_extractor in
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* ColumnFamilyOptions.
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*/
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class SliceTransform {
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public:
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virtual ~SliceTransform(){};
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// Return the name of this transformation.
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virtual const char* Name() const = 0;
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// Extract a prefix from a specified key. This method is called when
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// a key is inserted into the db, and the returned slice is used to
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// create a bloom filter.
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virtual Slice Transform(const Slice& key) const = 0;
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// Determine whether the specified key is compatible with the logic
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// specified in the Transform method. This method is invoked for every
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// key that is inserted into the db. If this method returns true,
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// then Transform is called to translate the key to its prefix and
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// that returned prefix is inserted into the bloom filter. If this
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// method returns false, then the call to Transform is skipped and
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// no prefix is inserted into the bloom filters.
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//
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// For example, if the Transform method operates on a fixed length
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// prefix of size 4, then an invocation to InDomain("abc") returns
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// false because the specified key length(3) is shorter than the
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// prefix size of 4.
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//
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// Wiki documentation here:
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// https://github.com/facebook/rocksdb/wiki/Prefix-Seek-API-Changes
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//
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virtual bool InDomain(const Slice& key) const = 0;
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// This is currently not used and remains here for backward compatibility.
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virtual bool InRange(const Slice& /*dst*/) const { return false; }
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// Some SliceTransform will have a full length which can be used to
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// determine if two keys are consecutive. Can be disabled by always
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// returning 0
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virtual bool FullLengthEnabled(size_t* /*len*/) const { return false; }
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// Transform(s)=Transform(`prefix`) for any s with `prefix` as a prefix.
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//
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// This function is not used by RocksDB, but for users. If users pass
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// Options by string to RocksDB, they might not know what prefix extractor
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// they are using. This function is to help users can determine:
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// if they want to iterate all keys prefixing `prefix`, whether it is
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// safe to use prefix bloom filter and seek to key `prefix`.
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// If this function returns true, this means a user can Seek() to a prefix
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// using the bloom filter. Otherwise, user needs to skip the bloom filter
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// by setting ReadOptions.total_order_seek = true.
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//
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// Here is an example: Suppose we implement a slice transform that returns
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// the first part of the string after splitting it using delimiter ",":
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// 1. SameResultWhenAppended("abc,") should return true. If applying prefix
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// bloom filter using it, all slices matching "abc:.*" will be extracted
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// to "abc,", so any SST file or memtable containing any of those key
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// will not be filtered out.
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// 2. SameResultWhenAppended("abc") should return false. A user will not
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// guaranteed to see all the keys matching "abc.*" if a user seek to "abc"
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// against a DB with the same setting. If one SST file only contains
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// "abcd,e", the file can be filtered out and the key will be invisible.
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//
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// i.e., an implementation always returning false is safe.
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virtual bool SameResultWhenAppended(const Slice& /*prefix*/) const {
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return false;
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}
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};
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extern const SliceTransform* NewFixedPrefixTransform(size_t prefix_len);
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extern const SliceTransform* NewCappedPrefixTransform(size_t cap_len);
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extern const SliceTransform* NewNoopTransform();
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} // namespace ROCKSDB_NAMESPACE
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