Summary: Here are the major changes to the Merge Interface. It has been expanded to handle cases where the MergeOperator is not associative. It does so by stacking up merge operations while scanning through the key history (i.e.: during Get() or Compaction), until a valid Put/Delete/end-of-history is encountered; it then applies all of the merge operations in the correct sequence starting with the base/sentinel value. I have also introduced an "AssociativeMerge" function which allows the user to take advantage of associative merge operations (such as in the case of counters). The implementation will always attempt to merge the operations/operands themselves together when they are encountered, and will resort to the "stacking" method if and only if the "associative-merge" fails. This implementation is conjectured to allow MergeOperator to handle the general case, while still providing the user with the ability to take advantage of certain efficiencies in their own merge-operator / data-structure. NOTE: This is a preliminary diff. This must still go through a lot of review, revision, and testing. Feedback welcome! Test Plan: -This is a preliminary diff. I have only just begun testing/debugging it. -I will be testing this with the existing MergeOperator use-cases and unit-tests (counters, string-append, and redis-lists) -I will be "desk-checking" and walking through the code with the help gdb. -I will find a way of stress-testing the new interface / implementation using db_bench, db_test, merge_test, and/or db_stress. -I will ensure that my tests cover all cases: Get-Memtable, Get-Immutable-Memtable, Get-from-Disk, Iterator-Range-Scan, Flush-Memtable-to-L0, Compaction-L0-L1, Compaction-Ln-L(n+1), Put/Delete found, Put/Delete not-found, end-of-history, end-of-file, etc. -A lot of feedback from the reviewers. Reviewers: haobo, dhruba, zshao, emayanke Reviewed By: haobo CC: leveldb Differential Revision: https://reviews.facebook.net/D11499
419 lines
13 KiB
C++
419 lines
13 KiB
C++
// Copyright (c) 2011 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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#include "table/table.h"
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#include "leveldb/cache.h"
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#include "leveldb/comparator.h"
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#include "leveldb/env.h"
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#include "leveldb/filter_policy.h"
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#include "leveldb/options.h"
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#include "leveldb/statistics.h"
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#include "table/block.h"
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#include "table/filter_block.h"
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#include "table/format.h"
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#include "table/two_level_iterator.h"
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#include "util/coding.h"
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#include "util/stop_watch.h"
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namespace leveldb {
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// The longest the prefix of the cache key used to identify blocks can be.
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// We are using the fact that we know for Posix files the unique ID is three
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// varints.
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const size_t kMaxCacheKeyPrefixSize = kMaxVarint64Length*3+1;
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struct Table::Rep {
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~Rep() {
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delete filter;
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delete [] filter_data;
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delete index_block;
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}
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Rep(const EnvOptions& storage_options) :
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soptions(storage_options) {
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}
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Options options;
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const EnvOptions& soptions;
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Status status;
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unique_ptr<RandomAccessFile> file;
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char cache_key_prefix[kMaxCacheKeyPrefixSize];
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size_t cache_key_prefix_size;
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FilterBlockReader* filter;
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const char* filter_data;
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BlockHandle metaindex_handle; // Handle to metaindex_block: saved from footer
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Block* index_block;
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};
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// Helper function to setup the cache key's prefix for the Table.
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void Table::SetupCacheKeyPrefix(Rep* rep) {
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assert(kMaxCacheKeyPrefixSize >= 10);
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rep->cache_key_prefix_size = 0;
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if (rep->options.block_cache) {
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rep->cache_key_prefix_size = rep->file->GetUniqueId(rep->cache_key_prefix,
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kMaxCacheKeyPrefixSize);
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if (rep->cache_key_prefix_size == 0) {
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// If the prefix wasn't generated or was too long, we create one from the
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// cache.
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char* end = EncodeVarint64(rep->cache_key_prefix,
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rep->options.block_cache->NewId());
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rep->cache_key_prefix_size =
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static_cast<size_t>(end - rep->cache_key_prefix);
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}
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}
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}
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namespace { // anonymous namespace, not visible externally
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// Read the block identified by "handle" from "file".
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// The only relevant option is options.verify_checksums for now.
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// Set *didIO to true if didIO is not null.
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// On failure return non-OK.
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// On success fill *result and return OK - caller owns *result
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Status ReadBlock(RandomAccessFile* file,
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const ReadOptions& options,
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const BlockHandle& handle,
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Block** result,
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bool* didIO = nullptr) {
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BlockContents contents;
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Status s = ReadBlockContents(file, options, handle, &contents);
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if (s.ok()) {
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*result = new Block(contents);
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}
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if (didIO) {
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*didIO = true;
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}
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return s;
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}
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} // end of anonymous namespace
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Status Table::Open(const Options& options,
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const EnvOptions& soptions,
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unique_ptr<RandomAccessFile>&& file,
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uint64_t size,
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unique_ptr<Table>* table) {
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table->reset();
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if (size < Footer::kEncodedLength) {
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return Status::InvalidArgument("file is too short to be an sstable");
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}
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char footer_space[Footer::kEncodedLength];
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Slice footer_input;
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Status s = file->Read(size - Footer::kEncodedLength, Footer::kEncodedLength,
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&footer_input, footer_space);
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if (!s.ok()) return s;
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// Check that we actually read the whole footer from the file. It may be
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// that size isn't correct.
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if (footer_input.size() != Footer::kEncodedLength) {
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return Status::InvalidArgument("file is too short to be an sstable");
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}
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Footer footer;
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s = footer.DecodeFrom(&footer_input);
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if (!s.ok()) return s;
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Block* index_block = nullptr;
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// TODO: we never really verify check sum for index block
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s = ReadBlock(file.get(), ReadOptions(), footer.index_handle(), &index_block);
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if (s.ok()) {
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// We've successfully read the footer and the index block: we're
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// ready to serve requests.
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Rep* rep = new Table::Rep(soptions);
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rep->options = options;
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rep->file = std::move(file);
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rep->metaindex_handle = footer.metaindex_handle();
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rep->index_block = index_block;
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SetupCacheKeyPrefix(rep);
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rep->filter_data = nullptr;
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rep->filter = nullptr;
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table->reset(new Table(rep));
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(*table)->ReadMeta(footer);
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} else {
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if (index_block) delete index_block;
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}
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return s;
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}
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void Table::SetupForCompaction() {
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switch (rep_->options.access_hint_on_compaction_start) {
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case Options::NONE:
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break;
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case Options::NORMAL:
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rep_->file->Hint(RandomAccessFile::NORMAL);
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break;
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case Options::SEQUENTIAL:
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rep_->file->Hint(RandomAccessFile::SEQUENTIAL);
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break;
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case Options::WILLNEED:
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rep_->file->Hint(RandomAccessFile::WILLNEED);
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break;
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default:
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assert(false);
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}
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compaction_optimized_ = true;
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}
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void Table::ReadMeta(const Footer& footer) {
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if (rep_->options.filter_policy == nullptr) {
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return; // Do not need any metadata
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}
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// TODO(sanjay): Skip this if footer.metaindex_handle() size indicates
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// it is an empty block.
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// TODO: we never really verify check sum for meta index block
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Block* meta = nullptr;
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if (!ReadBlock(rep_->file.get(), ReadOptions(), footer.metaindex_handle(),
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&meta).ok()) {
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// Do not propagate errors since meta info is not needed for operation
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return;
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}
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Iterator* iter = meta->NewIterator(BytewiseComparator());
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std::string key = "filter.";
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key.append(rep_->options.filter_policy->Name());
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iter->Seek(key);
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if (iter->Valid() && iter->key() == Slice(key)) {
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ReadFilter(iter->value());
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}
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delete iter;
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delete meta;
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}
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void Table::ReadFilter(const Slice& filter_handle_value) {
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Slice v = filter_handle_value;
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BlockHandle filter_handle;
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if (!filter_handle.DecodeFrom(&v).ok()) {
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return;
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}
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// TODO: We might want to unify with ReadBlock() if we start
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// requiring checksum verification in Table::Open.
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ReadOptions opt;
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BlockContents block;
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if (!ReadBlockContents(rep_->file.get(), opt, filter_handle, &block).ok()) {
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return;
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}
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if (block.heap_allocated) {
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rep_->filter_data = block.data.data(); // Will need to delete later
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}
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rep_->filter = new FilterBlockReader(rep_->options.filter_policy, block.data);
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}
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Table::~Table() {
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delete rep_;
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}
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static void DeleteBlock(void* arg, void* ignored) {
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delete reinterpret_cast<Block*>(arg);
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}
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static void DeleteCachedBlock(const Slice& key, void* value) {
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Block* block = reinterpret_cast<Block*>(value);
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delete block;
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}
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static void ReleaseBlock(void* arg, void* h) {
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Cache* cache = reinterpret_cast<Cache*>(arg);
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Cache::Handle* handle = reinterpret_cast<Cache::Handle*>(h);
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cache->Release(handle);
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}
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// Convert an index iterator value (i.e., an encoded BlockHandle)
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// into an iterator over the contents of the corresponding block.
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Iterator* Table::BlockReader(void* arg,
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const ReadOptions& options,
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const Slice& index_value,
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bool* didIO,
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bool for_compaction,
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const bool no_io) {
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Table* table = reinterpret_cast<Table*>(arg);
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Cache* block_cache = table->rep_->options.block_cache.get();
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std::shared_ptr<Statistics> statistics = table->rep_->options.statistics;
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Block* block = nullptr;
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Cache::Handle* cache_handle = nullptr;
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BlockHandle handle;
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Slice input = index_value;
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Status s = handle.DecodeFrom(&input);
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// We intentionally allow extra stuff in index_value so that we
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// can add more features in the future.
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if (s.ok()) {
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if (block_cache != nullptr) {
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char cache_key[kMaxCacheKeyPrefixSize + kMaxVarint64Length];
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const size_t cache_key_prefix_size = table->rep_->cache_key_prefix_size;
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assert(cache_key_prefix_size != 0);
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assert(cache_key_prefix_size <= kMaxCacheKeyPrefixSize);
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memcpy(cache_key, table->rep_->cache_key_prefix,
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cache_key_prefix_size);
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char* end = EncodeVarint64(cache_key + cache_key_prefix_size,
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handle.offset());
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Slice key(cache_key, static_cast<size_t>(end-cache_key));
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cache_handle = block_cache->Lookup(key);
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if (cache_handle != nullptr) {
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block = reinterpret_cast<Block*>(block_cache->Value(cache_handle));
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RecordTick(statistics, BLOCK_CACHE_HIT);
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} else if (no_io) {
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return nullptr; // Did not find in block_cache and can't do IO
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} else {
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Histograms histogram = for_compaction ?
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READ_BLOCK_COMPACTION_MICROS : READ_BLOCK_GET_MICROS;
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{ // block for stop watch
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StopWatch sw(table->rep_->options.env, statistics, histogram);
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s = ReadBlock(
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table->rep_->file.get(),
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options,
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handle,
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&block,
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didIO
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);
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}
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if (s.ok()) {
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if (block->isCachable() && options.fill_cache) {
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cache_handle = block_cache->Insert(
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key, block, block->size(), &DeleteCachedBlock);
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}
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}
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RecordTick(statistics, BLOCK_CACHE_MISS);
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}
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} else if (no_io) {
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return nullptr; // Could not read from block_cache and can't do IO
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}else {
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s = ReadBlock(table->rep_->file.get(), options, handle, &block, didIO);
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}
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}
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Iterator* iter;
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if (block != nullptr) {
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iter = block->NewIterator(table->rep_->options.comparator);
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if (cache_handle == nullptr) {
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iter->RegisterCleanup(&DeleteBlock, block, nullptr);
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} else {
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iter->RegisterCleanup(&ReleaseBlock, block_cache, cache_handle);
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}
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} else {
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iter = NewErrorIterator(s);
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}
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return iter;
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}
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Iterator* Table::BlockReader(void* arg,
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const ReadOptions& options,
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const EnvOptions& soptions,
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const Slice& index_value,
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bool for_compaction) {
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return BlockReader(arg, options, index_value, nullptr, for_compaction);
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}
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Iterator* Table::NewIterator(const ReadOptions& options) const {
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return NewTwoLevelIterator(
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rep_->index_block->NewIterator(rep_->options.comparator),
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&Table::BlockReader, const_cast<Table*>(this), options, rep_->soptions);
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}
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Status Table::InternalGet(const ReadOptions& options, const Slice& k,
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void* arg,
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bool (*saver)(void*, const Slice&, const Slice&,
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bool),
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void (*mark_key_may_exist)(void*),
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const bool no_io) {
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Status s;
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Iterator* iiter = rep_->index_block->NewIterator(rep_->options.comparator);
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bool done = false;
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for (iiter->Seek(k); iiter->Valid() && !done; iiter->Next()) {
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Slice handle_value = iiter->value();
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FilterBlockReader* filter = rep_->filter;
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BlockHandle handle;
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if (filter != nullptr &&
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handle.DecodeFrom(&handle_value).ok() &&
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!filter->KeyMayMatch(handle.offset(), k)) {
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// Not found
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// TODO: think about interaction with Merge. If a user key cannot
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// cross one data block, we should be fine.
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RecordTick(rep_->options.statistics, BLOOM_FILTER_USEFUL);
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break;
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} else {
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bool didIO = false;
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Iterator* block_iter = BlockReader(this, options, iiter->value(),
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&didIO, no_io);
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if (no_io && !block_iter) { // couldn't get block from block_cache
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// Update Saver.state to Found because we are only looking for whether
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// we can guarantee the key is not there when "no_io" is set
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(*mark_key_may_exist)(arg);
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break;
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}
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// Call the *saver function on each entry/block until it returns false
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for (block_iter->Seek(k); block_iter->Valid(); block_iter->Next()) {
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if (!(*saver)(arg, block_iter->key(), block_iter->value(), didIO)) {
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done = true;
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break;
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}
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}
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s = block_iter->status();
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delete block_iter;
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}
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}
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if (s.ok()) {
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s = iiter->status();
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}
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delete iiter;
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return s;
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}
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bool SaveDidIO(void* arg, const Slice& key, const Slice& value, bool didIO) {
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*reinterpret_cast<bool*>(arg) = didIO;
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return false;
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}
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bool Table::TEST_KeyInCache(const ReadOptions& options, const Slice& key) {
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// We use InternalGet() as it has logic that checks whether we read the
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// block from the disk or not.
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bool didIO = false;
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Status s = InternalGet(options, key, &didIO, SaveDidIO);
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assert(s.ok());
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return !didIO;
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}
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uint64_t Table::ApproximateOffsetOf(const Slice& key) const {
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Iterator* index_iter =
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rep_->index_block->NewIterator(rep_->options.comparator);
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index_iter->Seek(key);
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uint64_t result;
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if (index_iter->Valid()) {
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BlockHandle handle;
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Slice input = index_iter->value();
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Status s = handle.DecodeFrom(&input);
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if (s.ok()) {
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result = handle.offset();
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} else {
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// Strange: we can't decode the block handle in the index block.
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// We'll just return the offset of the metaindex block, which is
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// close to the whole file size for this case.
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result = rep_->metaindex_handle.offset();
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}
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} else {
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// key is past the last key in the file. Approximate the offset
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// by returning the offset of the metaindex block (which is
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// right near the end of the file).
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result = rep_->metaindex_handle.offset();
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}
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delete index_iter;
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return result;
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}
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} // namespace leveldb
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