root/chrome/browser/history/url_database.cc

DEFINITIONS

1. GetNextURL
2. GURLToDatabaseURL
3. FillURLRow
4. GetURLRow
5. GetAllTypedUrls
6. GetRowForURL
7. UpdateURLRow
8. AddURLInternal
9. InsertOrUpdateURLRowByID
10. DeleteURLRow
11. CreateTemporaryURLTable
12. CommitTemporaryURLTable
13. InitURLEnumeratorForEverything
14. InitURLEnumeratorForSignificant
15. AutocompleteForPrefix
16. IsTypedHost
17. FindShortestURLFromBase
18. GetTextMatches
19. InitKeywordSearchTermsTable
20. CreateKeywordSearchTermsIndices
21. DropKeywordSearchTermsTable
22. SetKeywordSearchTermsForURL
23. GetKeywordSearchTermRow
24. GetKeywordSearchTermRows
25. DeleteAllSearchTermsForKeyword
26. GetMostRecentKeywordSearchTerms
27. DeleteKeywordSearchTerm
28. DeleteKeywordSearchTermForURL
29. DropStarredIDFromURLs
30. CreateURLTable
31. CreateMainURLIndex

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "chrome/browser/history/url_database.h"

#include <algorithm>
#include <limits>
#include <string>
#include <vector>

#include "base/i18n/case_conversion.h"
#include "base/strings/utf_string_conversions.h"
#include "chrome/common/url_constants.h"
#include "net/base/net_util.h"
#include "sql/statement.h"
#include "ui/base/l10n/l10n_util.h"
#include "url/gurl.h"

namespace history {

const char URLDatabase::kURLRowFields[] = HISTORY_URL_ROW_FIELDS;
const int URLDatabase::kNumURLRowFields = 9;

URLDatabase::URLEnumeratorBase::URLEnumeratorBase()
    : initialized_(false) {
}

URLDatabase::URLEnumeratorBase::~URLEnumeratorBase() {
}

URLDatabase::URLEnumerator::URLEnumerator() {
}

bool URLDatabase::URLEnumerator::GetNextURL(URLRow* r) {
  if (statement_.Step()) {
    FillURLRow(statement_, r);
    return true;
  }
  return false;
}

URLDatabase::URLDatabase()
    : has_keyword_search_terms_(false) {
}

URLDatabase::~URLDatabase() {
}

// static
std::string URLDatabase::GURLToDatabaseURL(const GURL& gurl) {
  // TODO(brettw): do something fancy here with encoding, etc.

  // Strip username and password from URL before sending to DB.
  GURL::Replacements replacements;
  replacements.ClearUsername();
  replacements.ClearPassword();

  return (gurl.ReplaceComponents(replacements)).spec();
}

// Convenience to fill a history::URLRow. Must be in sync with the fields in
// kURLRowFields.
void URLDatabase::FillURLRow(sql::Statement& s, history::URLRow* i) {
  DCHECK(i);
  i->id_ = s.ColumnInt64(0);
  i->url_ = GURL(s.ColumnString(1));
  i->title_ = s.ColumnString16(2);
  i->visit_count_ = s.ColumnInt(3);
  i->typed_count_ = s.ColumnInt(4);
  i->last_visit_ = base::Time::FromInternalValue(s.ColumnInt64(5));
  i->hidden_ = s.ColumnInt(6) != 0;
}

bool URLDatabase::GetURLRow(URLID url_id, URLRow* info) {
  // TODO(brettw) We need check for empty URLs to handle the case where
  // there are old URLs in the database that are empty that got in before
  // we added any checks. We should eventually be able to remove it
  // when all inputs are using GURL (which prohibit empty input).
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls WHERE id=?"));
  statement.BindInt64(0, url_id);

  if (statement.Step()) {
    FillURLRow(statement, info);
    return true;
  }
  return false;
}

bool URLDatabase::GetAllTypedUrls(URLRows* urls) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls WHERE typed_count > 0"));

  while (statement.Step()) {
    URLRow info;
    FillURLRow(statement, &info);
    urls->push_back(info);
  }
  return true;
}

URLID URLDatabase::GetRowForURL(const GURL& url, history::URLRow* info) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls WHERE url=?"));
  std::string url_string = GURLToDatabaseURL(url);
  statement.BindString(0, url_string);

  if (!statement.Step())
    return 0;  // no data

  if (info)
    FillURLRow(statement, info);
  return statement.ColumnInt64(0);
}

bool URLDatabase::UpdateURLRow(URLID url_id,
                               const history::URLRow& info) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "UPDATE urls SET title=?,visit_count=?,typed_count=?,last_visit_time=?,"
        "hidden=?"
      "WHERE id=?"));
  statement.BindString16(0, info.title());
  statement.BindInt(1, info.visit_count());
  statement.BindInt(2, info.typed_count());
  statement.BindInt64(3, info.last_visit().ToInternalValue());
  statement.BindInt(4, info.hidden() ? 1 : 0);
  statement.BindInt64(5, url_id);

  return statement.Run();
}

URLID URLDatabase::AddURLInternal(const history::URLRow& info,
                                  bool is_temporary) {
  // This function is used to insert into two different tables, so we have to
  // do some shuffling. Unfortinately, we can't use the macro
  // HISTORY_URL_ROW_FIELDS because that specifies the table name which is
  // invalid in the insert syntax.
  #define ADDURL_COMMON_SUFFIX \
      " (url, title, visit_count, typed_count, "\
      "last_visit_time, hidden) "\
      "VALUES (?,?,?,?,?,?)"
  const char* statement_name;
  const char* statement_sql;
  if (is_temporary) {
    statement_name = "AddURLTemporary";
    statement_sql = "INSERT INTO temp_urls" ADDURL_COMMON_SUFFIX;
  } else {
    statement_name = "AddURL";
    statement_sql = "INSERT INTO urls" ADDURL_COMMON_SUFFIX;
  }
  #undef ADDURL_COMMON_SUFFIX

  sql::Statement statement(GetDB().GetCachedStatement(
      sql::StatementID(statement_name), statement_sql));
  statement.BindString(0, GURLToDatabaseURL(info.url()));
  statement.BindString16(1, info.title());
  statement.BindInt(2, info.visit_count());
  statement.BindInt(3, info.typed_count());
  statement.BindInt64(4, info.last_visit().ToInternalValue());
  statement.BindInt(5, info.hidden() ? 1 : 0);

  if (!statement.Run()) {
    VLOG(0) << "Failed to add url " << info.url().possibly_invalid_spec()
            << " to table history.urls.";
    return 0;
  }
  return GetDB().GetLastInsertRowId();
}

bool URLDatabase::InsertOrUpdateURLRowByID(const history::URLRow& info) {
  // SQLite does not support INSERT OR UPDATE, however, it does have INSERT OR
  // REPLACE, which is feasible to use, because of the following.
  //  * Before INSERTing, REPLACE will delete all pre-existing rows that cause
  //    constraint violations. Here, we only have a PRIMARY KEY constraint, so
  //    the only row that might get deleted is an old one with the same ID.
  //  * Another difference between the two flavors is that the latter actually
  //    deletes the old row, and thus the old values are lost in columns which
  //    are not explicitly assigned new values. This is not an issue, however,
  //    as we assign values to all columns.
  //  * When rows are deleted due to constraint violations, the delete triggers
  //    may not be invoked. As of now, we do not have any delete triggers.
  // For more details, see: http://www.sqlite.org/lang_conflict.html.
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "INSERT OR REPLACE INTO urls "
      "(id, url, title, visit_count, typed_count, last_visit_time, hidden) "
      "VALUES (?, ?, ?, ?, ?, ?, ?)"));

  statement.BindInt64(0, info.id());
  statement.BindString(1, GURLToDatabaseURL(info.url()));
  statement.BindString16(2, info.title());
  statement.BindInt(3, info.visit_count());
  statement.BindInt(4, info.typed_count());
  statement.BindInt64(5, info.last_visit().ToInternalValue());
  statement.BindInt(6, info.hidden() ? 1 : 0);

  return statement.Run();
}

bool URLDatabase::DeleteURLRow(URLID id) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "DELETE FROM urls WHERE id = ?"));
  statement.BindInt64(0, id);

  if (!statement.Run())
    return false;

  // And delete any keyword visits.
  return !has_keyword_search_terms_ || DeleteKeywordSearchTermForURL(id);
}

bool URLDatabase::CreateTemporaryURLTable() {
  return CreateURLTable(true);
}

bool URLDatabase::CommitTemporaryURLTable() {
  // See the comments in the header file as well as
  // HistoryBackend::DeleteAllHistory() for more information on how this works
  // and why it does what it does.
  //
  // Note that the main database overrides this to additionally create the
  // supplimentary indices that the archived database doesn't need.

  // Swap the url table out and replace it with the temporary one.
  if (!GetDB().Execute("DROP TABLE urls")) {
    NOTREACHED() << GetDB().GetErrorMessage();
    return false;
  }
  if (!GetDB().Execute("ALTER TABLE temp_urls RENAME TO urls")) {
    NOTREACHED() << GetDB().GetErrorMessage();
    return false;
  }

  // Create the index over URLs. This is needed for the main, in-memory, and
  // archived databases, so we always do it. The supplimentary indices used by
  // the main database are not created here. When deleting all history, they
  // are created by HistoryDatabase::RecreateAllButStarAndURLTables().
  CreateMainURLIndex();

  return true;
}

bool URLDatabase::InitURLEnumeratorForEverything(URLEnumerator* enumerator) {
  DCHECK(!enumerator->initialized_);
  std::string sql("SELECT ");
  sql.append(kURLRowFields);
  sql.append(" FROM urls");
  enumerator->statement_.Assign(GetDB().GetUniqueStatement(sql.c_str()));
  enumerator->initialized_ = enumerator->statement_.is_valid();
  return enumerator->statement_.is_valid();
}

bool URLDatabase::InitURLEnumeratorForSignificant(URLEnumerator* enumerator) {
  DCHECK(!enumerator->initialized_);
  std::string sql("SELECT ");
  sql.append(kURLRowFields);
  sql.append(" FROM urls WHERE last_visit_time >= ? OR visit_count >= ? OR "
             "typed_count >= ?");
  enumerator->statement_.Assign(GetDB().GetUniqueStatement(sql.c_str()));
  enumerator->statement_.BindInt64(
      0, AutocompleteAgeThreshold().ToInternalValue());
  enumerator->statement_.BindInt(1, kLowQualityMatchVisitLimit);
  enumerator->statement_.BindInt(2, kLowQualityMatchTypedLimit);
  enumerator->initialized_ = enumerator->statement_.is_valid();
  return enumerator->statement_.is_valid();
}

bool URLDatabase::AutocompleteForPrefix(const std::string& prefix,
                                        size_t max_results,
                                        bool typed_only,
                                        URLRows* results) {
  // NOTE: this query originally sorted by starred as the second parameter. But
  // as bookmarks is no longer part of the db we no longer include the order
  // by clause.
  results->clear();
  const char* sql;
  int line;
  if (typed_only) {
    sql = "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls "
        "WHERE url >= ? AND url < ? AND hidden = 0 AND typed_count > 0 "
        "ORDER BY typed_count DESC, visit_count DESC, last_visit_time DESC "
        "LIMIT ?";
    line = __LINE__;
  } else {
    sql = "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls "
        "WHERE url >= ? AND url < ? AND hidden = 0 "
        "ORDER BY typed_count DESC, visit_count DESC, last_visit_time DESC "
        "LIMIT ?";
    line = __LINE__;
  }
  sql::Statement statement(
      GetDB().GetCachedStatement(sql::StatementID(__FILE__, line), sql));

  // We will find all strings between "prefix" and this string, which is prefix
  // followed by the maximum character size. Use 8-bit strings for everything
  // so we can be sure sqlite is comparing everything in 8-bit mode. Otherwise,
  // it will have to convert strings either to UTF-8 or UTF-16 for comparison.
  std::string end_query(prefix);
  end_query.push_back(std::numeric_limits<unsigned char>::max());

  statement.BindString(0, prefix);
  statement.BindString(1, end_query);
  statement.BindInt(2, static_cast<int>(max_results));

  while (statement.Step()) {
    history::URLRow info;
    FillURLRow(statement, &info);
    if (info.url().is_valid())
      results->push_back(info);
  }
  return !results->empty();
}

bool URLDatabase::IsTypedHost(const std::string& host) {
  const char* schemes[] = {
    content::kHttpScheme,
    content::kHttpsScheme,
    content::kFtpScheme
  };
  URLRows dummy;
  for (size_t i = 0; i < arraysize(schemes); ++i) {
    std::string scheme_and_host(schemes[i]);
    scheme_and_host += content::kStandardSchemeSeparator + host;
    if (AutocompleteForPrefix(scheme_and_host + '/', 1, true, &dummy) ||
        AutocompleteForPrefix(scheme_and_host + ':', 1, true, &dummy))
      return true;
  }
  return false;
}

bool URLDatabase::FindShortestURLFromBase(const std::string& base,
                                          const std::string& url,
                                          int min_visits,
                                          int min_typed,
                                          bool allow_base,
                                          history::URLRow* info) {
  // Select URLs that start with |base| and are prefixes of |url|.  All parts
  // of this query except the substr() call can be done using the index.  We
  // could do this query with a couple of LIKE or GLOB statements as well, but
  // those wouldn't use the index, and would run into problems with "wildcard"
  // characters that appear in URLs (% for LIKE, or *, ? for GLOB).
  std::string sql("SELECT ");
  sql.append(kURLRowFields);
  sql.append(" FROM urls WHERE url ");
  sql.append(allow_base ? ">=" : ">");
  sql.append(" ? AND url < :end AND url = substr(:end, 1, length(url)) "
             "AND hidden = 0 AND visit_count >= ? AND typed_count >= ? "
             "ORDER BY url LIMIT 1");
  sql::Statement statement(GetDB().GetUniqueStatement(sql.c_str()));
  statement.BindString(0, base);
  statement.BindString(1, url);   // :end
  statement.BindInt(2, min_visits);
  statement.BindInt(3, min_typed);

  if (!statement.Step())
    return false;

  DCHECK(info);
  FillURLRow(statement, info);
  return true;
}

bool URLDatabase::GetTextMatches(const base::string16& query,
                                 URLRows* results) {
  ScopedVector<QueryNode> query_nodes;
  query_parser_.ParseQueryNodes(query, &query_nodes.get());

  results->clear();
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT" HISTORY_URL_ROW_FIELDS "FROM urls WHERE hidden = 0"));

  while (statement.Step()) {
    std::vector<QueryWord> query_words;
    base::string16 url = base::i18n::ToLower(statement.ColumnString16(1));
    query_parser_.ExtractQueryWords(url, &query_words);
    GURL gurl(url);
    if (gurl.is_valid()) {
      // Decode punycode to match IDN.
      // |query_words| won't be shown to user - therefore we can use empty
      // |languages| to reduce dependency (no need to call PrefService).
      base::string16 ascii = base::ASCIIToUTF16(gurl.host());
      base::string16 utf = net::IDNToUnicode(gurl.host(), std::string());
      if (ascii != utf)
        query_parser_.ExtractQueryWords(utf, &query_words);
    }
    base::string16 title = base::i18n::ToLower(statement.ColumnString16(2));
    query_parser_.ExtractQueryWords(title, &query_words);

    if (query_parser_.DoesQueryMatch(query_words, query_nodes.get())) {
      history::URLResult info;
      FillURLRow(statement, &info);
      if (info.url().is_valid())
        results->push_back(info);
    }
  }
  return !results->empty();
}

bool URLDatabase::InitKeywordSearchTermsTable() {
  has_keyword_search_terms_ = true;
  if (!GetDB().DoesTableExist("keyword_search_terms")) {
    if (!GetDB().Execute("CREATE TABLE keyword_search_terms ("
        "keyword_id INTEGER NOT NULL,"      // ID of the TemplateURL.
        "url_id INTEGER NOT NULL,"          // ID of the url.
        "lower_term LONGVARCHAR NOT NULL,"  // The search term, in lower case.
        "term LONGVARCHAR NOT NULL)"))      // The actual search term.
      return false;
  }
  return true;
}

bool URLDatabase::CreateKeywordSearchTermsIndices() {
  // For searching.
  if (!GetDB().Execute(
          "CREATE INDEX IF NOT EXISTS keyword_search_terms_index1 ON "
          "keyword_search_terms (keyword_id, lower_term)")) {
    return false;
  }

  // For deletion.
  if (!GetDB().Execute(
          "CREATE INDEX IF NOT EXISTS keyword_search_terms_index2 ON "
          "keyword_search_terms (url_id)")) {
    return false;
  }

  // For query or deletion by term.
  if (!GetDB().Execute(
          "CREATE INDEX IF NOT EXISTS keyword_search_terms_index3 ON "
          "keyword_search_terms (term)")) {
    return false;
  }
  return true;
}

bool URLDatabase::DropKeywordSearchTermsTable() {
  // This will implicitly delete the indices over the table.
  return GetDB().Execute("DROP TABLE keyword_search_terms");
}

bool URLDatabase::SetKeywordSearchTermsForURL(URLID url_id,
                                              TemplateURLID keyword_id,
                                              const base::string16& term) {
  DCHECK(url_id && keyword_id && !term.empty());

  sql::Statement exist_statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT term FROM keyword_search_terms "
      "WHERE keyword_id = ? AND url_id = ?"));
  exist_statement.BindInt64(0, keyword_id);
  exist_statement.BindInt64(1, url_id);

  if (exist_statement.Step())
    return true;  // Term already exists, no need to add it.

  if (!exist_statement.Succeeded())
    return false;

  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "INSERT INTO keyword_search_terms (keyword_id, url_id, lower_term, term) "
      "VALUES (?,?,?,?)"));
  statement.BindInt64(0, keyword_id);
  statement.BindInt64(1, url_id);
  statement.BindString16(2, base::i18n::ToLower(term));
  statement.BindString16(3, term);
  return statement.Run();
}

bool URLDatabase::GetKeywordSearchTermRow(URLID url_id,
                                          KeywordSearchTermRow* row) {
  DCHECK(url_id);
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT keyword_id, term FROM keyword_search_terms WHERE url_id=?"));
  statement.BindInt64(0, url_id);

  if (!statement.Step())
    return false;

  if (row) {
    row->url_id = url_id;
    row->keyword_id = statement.ColumnInt64(0);
    row->term = statement.ColumnString16(1);
  }
  return true;
}

bool URLDatabase::GetKeywordSearchTermRows(
    const base::string16& term,
    std::vector<KeywordSearchTermRow>* rows) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT keyword_id, url_id FROM keyword_search_terms WHERE term=?"));
  statement.BindString16(0, term);

  if (!statement.is_valid())
    return false;

  while (statement.Step()) {
    KeywordSearchTermRow row;
    row.url_id = statement.ColumnInt64(1);
    row.keyword_id = statement.ColumnInt64(0);
    row.term = term;
    rows->push_back(row);
  }
  return true;
}

void URLDatabase::DeleteAllSearchTermsForKeyword(
    TemplateURLID keyword_id) {
  DCHECK(keyword_id);
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "DELETE FROM keyword_search_terms WHERE keyword_id=?"));
  statement.BindInt64(0, keyword_id);

  statement.Run();
}

void URLDatabase::GetMostRecentKeywordSearchTerms(
    TemplateURLID keyword_id,
    const base::string16& prefix,
    int max_count,
    std::vector<KeywordSearchTermVisit>* matches) {
  // NOTE: the keyword_id can be zero if on first run the user does a query
  // before the TemplateURLService has finished loading. As the chances of this
  // occurring are small, we ignore it.
  if (!keyword_id)
    return;

  DCHECK(!prefix.empty());
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "SELECT DISTINCT kv.term, u.visit_count, u.last_visit_time "
      "FROM keyword_search_terms kv "
      "JOIN urls u ON kv.url_id = u.id "
      "WHERE kv.keyword_id = ? AND kv.lower_term >= ? AND kv.lower_term < ? "
      "ORDER BY u.last_visit_time DESC LIMIT ?"));

  // NOTE: Keep this ToLower() call in sync with search_provider.cc.
  base::string16 lower_prefix = base::i18n::ToLower(prefix);
  // This magic gives us a prefix search.
  base::string16 next_prefix = lower_prefix;
  next_prefix[next_prefix.size() - 1] =
      next_prefix[next_prefix.size() - 1] + 1;
  statement.BindInt64(0, keyword_id);
  statement.BindString16(1, lower_prefix);
  statement.BindString16(2, next_prefix);
  statement.BindInt(3, max_count);

  KeywordSearchTermVisit visit;
  while (statement.Step()) {
    visit.term = statement.ColumnString16(0);
    visit.visits = statement.ColumnInt(1);
    visit.time = base::Time::FromInternalValue(statement.ColumnInt64(2));
    matches->push_back(visit);
  }
}

bool URLDatabase::DeleteKeywordSearchTerm(const base::string16& term) {
  sql::Statement statement(GetDB().GetCachedStatement(SQL_FROM_HERE,
      "DELETE FROM keyword_search_terms WHERE term=?"));
  statement.BindString16(0, term);

  return statement.Run();
}

bool URLDatabase::DeleteKeywordSearchTermForURL(URLID url_id) {
  sql::Statement statement(GetDB().GetCachedStatement(
      SQL_FROM_HERE, "DELETE FROM keyword_search_terms WHERE url_id=?"));
  statement.BindInt64(0, url_id);
  return statement.Run();
}

bool URLDatabase::DropStarredIDFromURLs() {
  if (!GetDB().DoesColumnExist("urls", "starred_id"))
    return true;  // urls is already updated, no need to continue.

  // Create a temporary table to contain the new URLs table.
  if (!CreateTemporaryURLTable()) {
    NOTREACHED();
    return false;
  }

  // Copy the contents.
  if (!GetDB().Execute(
      "INSERT INTO temp_urls (id, url, title, visit_count, typed_count, "
      "last_visit_time, hidden, favicon_id) "
      "SELECT id, url, title, visit_count, typed_count, last_visit_time, "
      "hidden, favicon_id FROM urls")) {
    NOTREACHED() << GetDB().GetErrorMessage();
    return false;
  }

  // Rename/commit the tmp table.
  CommitTemporaryURLTable();

  return true;
}

bool URLDatabase::CreateURLTable(bool is_temporary) {
  const char* name = is_temporary ? "temp_urls" : "urls";
  if (GetDB().DoesTableExist(name))
    return true;

  // Note: revise implementation for InsertOrUpdateURLRowByID() if you add any
  // new constraints to the schema.
  std::string sql;
  sql.append("CREATE TABLE ");
  sql.append(name);
  sql.append("("
      "id INTEGER PRIMARY KEY,"
      "url LONGVARCHAR,"
      "title LONGVARCHAR,"
      "visit_count INTEGER DEFAULT 0 NOT NULL,"
      "typed_count INTEGER DEFAULT 0 NOT NULL,"
      "last_visit_time INTEGER NOT NULL,"
      "hidden INTEGER DEFAULT 0 NOT NULL,"
      "favicon_id INTEGER DEFAULT 0 NOT NULL)"); // favicon_id is not used now.

  return GetDB().Execute(sql.c_str());
}

bool URLDatabase::CreateMainURLIndex() {
  // Index over URLs so we can quickly look up based on URL.
  return GetDB().Execute(
      "CREATE INDEX IF NOT EXISTS urls_url_index ON urls (url)");
}

}  // namespace history