components/history/core/browser/url_database.h - chromium/src - Git at Google

 // Copyright 2014 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.

 #ifndef COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_

 #include <stddef.h>

 #include <string>
 #include <vector>

 #include "base/time/time.h"
 #include "components/history/core/browser/keyword_id.h"
 #include "components/history/core/browser/url_row.h"
 #include "components/query_parser/query_parser.h"
 #include "sql/statement.h"

 class GURL;

 namespace base {
 class Time;
 }

 namespace sql {
 class Database;
 }

 namespace history {

 struct KeywordSearchTermRow;
 struct KeywordSearchTermVisit;
 struct NormalizedKeywordSearchTermVisit;

 class VisitDatabase;  // For friend statement.

 // Encapsulates an SQL database that holds URL info.  This is a subset of the
 // full history data.  We split this class' functionality out from the larger
 // HistoryDatabase class to support maintaining separate databases of URLs with
 // different capabilities (for example, the in-memory database).
 //
 // This is refcounted to support calling InvokeLater() with some of its methods
 // (necessary to maintain ordering of DB operations).
 class URLDatabase {
  public:
   // Must call CreateURLTable() and CreateURLIndexes() before using to make
   // sure the database is initialized.
   URLDatabase();

   URLDatabase(const URLDatabase&) = delete;
   URLDatabase& operator=(const URLDatabase&) = delete;

   // This object must be destroyed on the thread where all accesses are
   // happening to avoid thread-safety problems.
   virtual ~URLDatabase();

   // URL table functions -------------------------------------------------------

   // Looks up a url given an id. Fills info with the data. Returns true on
   // success and false otherwise.
   bool GetURLRow(URLID url_id, URLRow* info);

   // Looks up the given URL and if it exists, fills the given pointers with the
   // associated info and returns the ID of that URL. If the info pointer is
   // NULL, no information about the URL will be filled in, only the ID will be
   // returned. Returns 0 if the URL was not found.
   URLID GetRowForURL(const GURL& url, URLRow* info);

   // Given an already-existing row in the URL table, updates that URL's stats.
   // This can not change the URL.  Returns true on success.
   //
   // This will NOT update the title used for full text indexing. If you are
   // setting the title, call SetPageIndexedData with the new title.
   bool UpdateURLRow(URLID url_id, const URLRow& info);

   // Adds a line to the URL database with the given information and returns the
   // newly generated ID for the row (the `id` in `info` is ignored). A row with
   // the given URL must not exist. Returns 0 on error.
   //
   // This does NOT add a row to the full text search database. Use
   // HistoryDatabase::SetPageIndexedData to do this.
   URLID AddURL(const URLRow& info) {
     return AddURLInternal(info, false);
   }

   // Either adds a new row to the URL table with the given information (with the
   // the `id` as specified in `info`), or updates the pre-existing row with this
   // `id` if there is one already. This is also known as an "upsert" or "merge"
   // operation. Returns true on success.
   bool InsertOrUpdateURLRowByID(const URLRow& info);

   // Delete the row of the corresponding URL. Only the row in the URL table and
   // corresponding keyword search terms will be deleted, not any other data that
   // may refer to the URL row. Returns true if the row existed and was deleted.
   bool DeleteURLRow(URLID id);

   // URL mass-deleting ---------------------------------------------------------

   // Begins the mass-deleting operation by creating a temporary URL table.
   // The caller than adds the URLs it wants to preserve to the temporary table,
   // and then deletes everything else by calling CommitTemporaryURLTable().
   // Returns true on success.
   bool CreateTemporaryURLTable();

   // Adds a row to the temporary URL table. This must be called between
   // CreateTemporaryURLTable() and CommitTemporaryURLTable() (see those for more
   // info). The ID of the URL will change in the temporary table, so the new ID
   // is returned. Returns 0 on failure.
   URLID AddTemporaryURL(const URLRow& row) {
     return AddURLInternal(row, true);
   }

   // Ends the mass-deleting by replacing the original URL table with the
   // temporary one created in CreateTemporaryURLTable. Returns true on success.
   bool CommitTemporaryURLTable();

   // Enumeration ---------------------------------------------------------------

   // A basic enumerator to enumerate urls database.
   class URLEnumeratorBase {
    public:
     URLEnumeratorBase();

     URLEnumeratorBase(const URLEnumeratorBase&) = delete;
     URLEnumeratorBase& operator=(const URLEnumeratorBase&) = delete;

     virtual ~URLEnumeratorBase();

    private:
     friend class URLDatabase;

     bool initialized_;
     sql::Statement statement_;
   };

   // A basic enumerator to enumerate urls
   class URLEnumerator : public URLEnumeratorBase {
    public:
     URLEnumerator();

     URLEnumerator(const URLEnumerator&) = delete;
     URLEnumerator& operator=(const URLEnumerator&) = delete;

     // Retrieves the next url. Returns false if no more urls are available.
     bool GetNextURL(URLRow* r);
   };

   // Initializes the given enumerator to enumerator all URLs in the database.
   bool InitURLEnumeratorForEverything(URLEnumerator* enumerator);

   // Initializes the given enumerator to enumerator all URLs in the database
   // that are historically significant: ones having their URL manually typed
   // more than once, having been visited within 3 days, or having been visited
   // more than 3 times in the order of the most significant ones first.
   bool InitURLEnumeratorForSignificant(URLEnumerator* enumerator);

   // Autocomplete --------------------------------------------------------------

   // Fills the given array with URLs matching the given prefix.  They will be
   // sorted by typed count, then by visit count, then by visit date (most recent
   // first) up to the given maximum number.  If `typed_only` is true, only urls
   // that have been typed once are returned.  For caller convenience, returns
   // whether any results were found.
   bool AutocompleteForPrefix(const std::string& prefix,
                              size_t max_results,
                              bool typed_only,
                              URLRows* results);

   // Returns true if the database holds some past typed navigation to a URL on
   // the provided hostname. If the return value is true and `scheme` is not
   // nullptr, `scheme` holds the scheme of one of the corresponding entries in
   // the database.
   bool IsTypedHost(const std::string& host, std::string* scheme);

   // Tries to find the shortest URL beginning with `base` that strictly
   // prefixes `url`, and has minimum visit_ and typed_counts as specified.
   // If found, fills in `info` and returns true; otherwise returns false,
   // leaving `info` unchanged.
   // We allow matches of exactly `base` iff `allow_base` is true.
   bool FindShortestURLFromBase(const std::string& base,
                                const std::string& url,
                                int min_visits,
                                int min_typed,
                                bool allow_base,
                                URLRow* info);

   // History search ------------------------------------------------------------

   // Performs a brute force search over the database to find any URLs or titles
   // which match the `query` string, using the default text matching algorithm.
   // Returns any matches.
   URLRows GetTextMatches(const std::u16string& query);

   // Same as GetTextMatches, using `algorithm` as the text matching
   // algorithm.
   URLRows GetTextMatchesWithAlgorithm(
       const std::u16string& query,
       query_parser::MatchingAlgorithm algorithm);

   // Keyword Search Terms ------------------------------------------------------

   // Sets the search terms for the specified url/keyword pair.
   bool SetKeywordSearchTermsForURL(URLID url_id,
                                    KeywordID keyword_id,
                                    const std::u16string& term);

   // Looks up a keyword search term given a url id. Returns all the search terms
   // in `rows`. Returns true on success.
   bool GetKeywordSearchTermRow(URLID url_id, KeywordSearchTermRow* row);

   // Looks up all keyword search terms given a term, Fills the rows with data.
   // Returns true on success and false otherwise.
   bool GetKeywordSearchTermRows(const std::u16string& term,
                                 std::vector<KeywordSearchTermRow>* rows);

   // Deletes all search terms for the specified keyword that have been added by
   // way of SetKeywordSearchTermsForURL.
   void DeleteAllSearchTermsForKeyword(KeywordID keyword_id);

   // Returns up to max_count of the most recent search terms for the specified
   // keyword.
   void GetMostRecentKeywordSearchTerms(
       KeywordID keyword_id,
       const std::u16string& prefix,
       int max_count,
       std::vector<KeywordSearchTermVisit>* matches);

   // Returns the most recent (i.e., no older than `age_threshold`) normalized
   // search terms (i.e., search terms in lower case with whitespaces collapsed)
   // for the specified keyword.
   std::vector<NormalizedKeywordSearchTermVisit>
   GetMostRecentNormalizedKeywordSearchTerms(KeywordID keyword_id,
                                             base::Time age_threshold);

   // Deletes all searches matching `term`.
   bool DeleteKeywordSearchTerm(const std::u16string& term);

   // Deletes any search corresponding to `normalized_term`.
   bool DeleteKeywordSearchTermForNormalizedTerm(
       KeywordID keyword_id,
       const std::u16string& normalized_term);

   // Deletes any search corresponding to `url_id`.
   bool DeleteKeywordSearchTermForURL(URLID url_id);

  protected:
   friend class VisitDatabase;

   // See HISTORY_URL_ROW_FIELDS below.
   static const char kURLRowFields[];

   // The number of fiends in kURLRowFields. If callers need additional
   // fields, they can add their 0-based index to this value to get the index of
   // fields following kURLRowFields.
   static const int kNumURLRowFields;

   // Drops the starred_id column from urls, returning true on success. This does
   // nothing (and returns true) if the urls doesn't contain the starred_id
   // column.
   bool DropStarredIDFromURLs();

   // Initialization functions. The indexing functions are separate from the
   // table creation functions so the in-memory database and the temporary tables
   // used when clearing history can populate the table and then create the
   // index, which is faster than the reverse.
   //
   // is_temporary is false when generating the "regular" URLs table. The expirer
   // sets this to true to generate the temporary table, which will have a
   // different name but the same schema.
   bool CreateURLTable(bool is_temporary);

   // Creates the index over URLs so we can quickly look up based on URL.
   bool CreateMainURLIndex();

   // Recreate URL table, and keep all existing contents.
   bool RecreateURLTableWithAllContents();

   // Ensures the keyword search terms table exists.
   bool InitKeywordSearchTermsTable();

   // Creates the indices used for keyword search terms.
   bool CreateKeywordSearchTermsIndices();

   // Deletes the keyword search terms table.
   bool DropKeywordSearchTermsTable();

   // Inserts the given URL row into the URLs table, using the regular table
   // if is_temporary is false, or the temporary URL table if is temporary is
   // true. The current `id` of `info` will be ignored in both cases and a new ID
   // will be generated, which will also constitute the return value, except in
   // case of an error, when the return value is 0. The temporary table may only
   // be used in between CreateTemporaryURLTable() and CommitTemporaryURLTable().
   URLID AddURLInternal(const URLRow& info, bool is_temporary);

   // Return ture if the urls table's schema contains "AUTOINCREMENT".
   // false if table do not contain AUTOINCREMENT, or the table is not created.
   bool URLTableContainsAutoincrement();

   // Convenience to fill a URLRow. Must be in sync with the fields in
   // kHistoryURLRowFields.
   static void FillURLRow(sql::Statement& s, URLRow* i);

   // Returns the database for the functions in this interface. The descendant of
   // this class implements these functions to return its objects.
   virtual sql::Database& GetDB() = 0;

   // Replaces the lower_term column in the keyword search terms table with
   // normalized_term which contains the search term, in lower case, and with
   // whitespaces collapsed for migration to version 42.
   bool MigrateKeywordsSearchTermsLowerTermColumn();

  private:
   // True if InitKeywordSearchTermsTable() has been invoked. Not all subclasses
   // have keyword search terms.
   bool has_keyword_search_terms_;
 };

 // The fields and order expected by FillURLRow(). ID is guaranteed to be first
 // so that DISTINCT can be prepended to get distinct URLs.
 //
 // This is available BOTH as a macro and a static string (kURLRowFields). Use
 // the macro if you want to put this in the middle of an otherwise constant
 // string, it will save time doing string appends. If you have to build a SQL
 // string dynamically anyway, use the constant, it will save space.
 #define HISTORY_URL_ROW_FIELDS \
     " urls.id, urls.url, urls.title, urls.visit_count, urls.typed_count, " \
     "urls.last_visit_time, urls.hidden "

 // Constants which specify, when considered altogether, 'significant'
 // history items. These are used to filter out insignificant items
 // for consideration as autocomplete candidates.
 extern const int kLowQualityMatchTypedLimit;
 extern const int kLowQualityMatchVisitLimit;
 extern const int kLowQualityMatchAgeLimitInDays;

 // The time interval within which a duplicate query is considered invalid for
 // autocomplete purposes.
 // These invalid duplicates are extracted from search query URLs which are
 // identical or nearly identical to the original search query URL and issued too
 // closely to it, i.e., within this time interval. They are typically recorded
 // as a result of back/forward navigations or user interactions in the search
 // result page and are likely not newly initiated searches.
 extern const base::TimeDelta kAutocompleteDuplicateVisitIntervalThreshold;

 // Returns the date threshold for considering an history item as significant.
 base::Time AutocompleteAgeThreshold();

 // Return true if `row` qualifies as an autocomplete candidate. If `threshold`
 // is_null() then this function determines a new time threshold each time it is
 // called. Since getting system time can be costly (such as for cases where
 // this function will be called in a loop over many history items), you can
 // provide a non-null `threshold` by simply initializing `threshold` with
 // AutocompleteAgeThreshold() (or any other desired time in the past).
 bool RowQualifiesAsSignificant(const URLRow& row, const base::Time& threshold);

 }  // namespace history

 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_
	// Copyright 2014 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.

	#ifndef COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_
	#define COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_

	#include <stddef.h>

	#include <string>
	#include <vector>

	#include "base/time/time.h"
	#include "components/history/core/browser/keyword_id.h"
	#include "components/history/core/browser/url_row.h"
	#include "components/query_parser/query_parser.h"
	#include "sql/statement.h"

	class GURL;

	namespace base {
	class Time;
	}

	namespace sql {
	class Database;
	}

	namespace history {

	struct KeywordSearchTermRow;
	struct KeywordSearchTermVisit;
	struct NormalizedKeywordSearchTermVisit;

	class VisitDatabase; // For friend statement.

	// Encapsulates an SQL database that holds URL info. This is a subset of the
	// full history data. We split this class' functionality out from the larger
	// HistoryDatabase class to support maintaining separate databases of URLs with
	// different capabilities (for example, the in-memory database).
	//
	// This is refcounted to support calling InvokeLater() with some of its methods
	// (necessary to maintain ordering of DB operations).
	class URLDatabase {
	public:
	// Must call CreateURLTable() and CreateURLIndexes() before using to make
	// sure the database is initialized.
	URLDatabase();

	URLDatabase(const URLDatabase&) = delete;
	URLDatabase& operator=(const URLDatabase&) = delete;

	// This object must be destroyed on the thread where all accesses are
	// happening to avoid thread-safety problems.
	virtual ~URLDatabase();

	// URL table functions -------------------------------------------------------

	// Looks up a url given an id. Fills info with the data. Returns true on
	// success and false otherwise.
	bool GetURLRow(URLID url_id, URLRow* info);

	// Looks up the given URL and if it exists, fills the given pointers with the
	// associated info and returns the ID of that URL. If the info pointer is
	// NULL, no information about the URL will be filled in, only the ID will be
	// returned. Returns 0 if the URL was not found.
	URLID GetRowForURL(const GURL& url, URLRow* info);

	// Given an already-existing row in the URL table, updates that URL's stats.
	// This can not change the URL. Returns true on success.
	//
	// This will NOT update the title used for full text indexing. If you are
	// setting the title, call SetPageIndexedData with the new title.
	bool UpdateURLRow(URLID url_id, const URLRow& info);

	// Adds a line to the URL database with the given information and returns the
	// newly generated ID for the row (the `id` in `info` is ignored). A row with
	// the given URL must not exist. Returns 0 on error.
	//
	// This does NOT add a row to the full text search database. Use
	// HistoryDatabase::SetPageIndexedData to do this.
	URLID AddURL(const URLRow& info) {
	return AddURLInternal(info, false);
	}

	// Either adds a new row to the URL table with the given information (with the
	// the `id` as specified in `info`), or updates the pre-existing row with this
	// `id` if there is one already. This is also known as an "upsert" or "merge"
	// operation. Returns true on success.
	bool InsertOrUpdateURLRowByID(const URLRow& info);

	// Delete the row of the corresponding URL. Only the row in the URL table and
	// corresponding keyword search terms will be deleted, not any other data that
	// may refer to the URL row. Returns true if the row existed and was deleted.
	bool DeleteURLRow(URLID id);

	// URL mass-deleting ---------------------------------------------------------

	// Begins the mass-deleting operation by creating a temporary URL table.
	// The caller than adds the URLs it wants to preserve to the temporary table,
	// and then deletes everything else by calling CommitTemporaryURLTable().
	// Returns true on success.
	bool CreateTemporaryURLTable();

	// Adds a row to the temporary URL table. This must be called between
	// CreateTemporaryURLTable() and CommitTemporaryURLTable() (see those for more
	// info). The ID of the URL will change in the temporary table, so the new ID
	// is returned. Returns 0 on failure.
	URLID AddTemporaryURL(const URLRow& row) {
	return AddURLInternal(row, true);
	}

	// Ends the mass-deleting by replacing the original URL table with the
	// temporary one created in CreateTemporaryURLTable. Returns true on success.
	bool CommitTemporaryURLTable();

	// Enumeration ---------------------------------------------------------------

	// A basic enumerator to enumerate urls database.
	class URLEnumeratorBase {
	public:
	URLEnumeratorBase();

	URLEnumeratorBase(const URLEnumeratorBase&) = delete;
	URLEnumeratorBase& operator=(const URLEnumeratorBase&) = delete;

	virtual ~URLEnumeratorBase();

	private:
	friend class URLDatabase;

	bool initialized_;
	sql::Statement statement_;
	};

	// A basic enumerator to enumerate urls
	class URLEnumerator : public URLEnumeratorBase {
	public:
	URLEnumerator();

	URLEnumerator(const URLEnumerator&) = delete;
	URLEnumerator& operator=(const URLEnumerator&) = delete;

	// Retrieves the next url. Returns false if no more urls are available.
	bool GetNextURL(URLRow* r);
	};

	// Initializes the given enumerator to enumerator all URLs in the database.
	bool InitURLEnumeratorForEverything(URLEnumerator* enumerator);

	// Initializes the given enumerator to enumerator all URLs in the database
	// that are historically significant: ones having their URL manually typed
	// more than once, having been visited within 3 days, or having been visited
	// more than 3 times in the order of the most significant ones first.
	bool InitURLEnumeratorForSignificant(URLEnumerator* enumerator);

	// Autocomplete --------------------------------------------------------------

	// Fills the given array with URLs matching the given prefix. They will be
	// sorted by typed count, then by visit count, then by visit date (most recent
	// first) up to the given maximum number. If `typed_only` is true, only urls
	// that have been typed once are returned. For caller convenience, returns
	// whether any results were found.
	bool AutocompleteForPrefix(const std::string& prefix,
	size_t max_results,
	bool typed_only,
	URLRows* results);

	// Returns true if the database holds some past typed navigation to a URL on
	// the provided hostname. If the return value is true and `scheme` is not
	// nullptr, `scheme` holds the scheme of one of the corresponding entries in
	// the database.
	bool IsTypedHost(const std::string& host, std::string* scheme);

	// Tries to find the shortest URL beginning with `base` that strictly
	// prefixes `url`, and has minimum visit_ and typed_counts as specified.
	// If found, fills in `info` and returns true; otherwise returns false,
	// leaving `info` unchanged.
	// We allow matches of exactly `base` iff `allow_base` is true.
	bool FindShortestURLFromBase(const std::string& base,
	const std::string& url,
	int min_visits,
	int min_typed,
	bool allow_base,
	URLRow* info);

	// History search ------------------------------------------------------------

	// Performs a brute force search over the database to find any URLs or titles
	// which match the `query` string, using the default text matching algorithm.
	// Returns any matches.
	URLRows GetTextMatches(const std::u16string& query);

	// Same as GetTextMatches, using `algorithm` as the text matching
	// algorithm.
	URLRows GetTextMatchesWithAlgorithm(
	const std::u16string& query,
	query_parser::MatchingAlgorithm algorithm);

	// Keyword Search Terms ------------------------------------------------------

	// Sets the search terms for the specified url/keyword pair.
	bool SetKeywordSearchTermsForURL(URLID url_id,
	KeywordID keyword_id,
	const std::u16string& term);

	// Looks up a keyword search term given a url id. Returns all the search terms
	// in `rows`. Returns true on success.
	bool GetKeywordSearchTermRow(URLID url_id, KeywordSearchTermRow* row);

	// Looks up all keyword search terms given a term, Fills the rows with data.
	// Returns true on success and false otherwise.
	bool GetKeywordSearchTermRows(const std::u16string& term,
	std::vector<KeywordSearchTermRow>* rows);

	// Deletes all search terms for the specified keyword that have been added by
	// way of SetKeywordSearchTermsForURL.
	void DeleteAllSearchTermsForKeyword(KeywordID keyword_id);

	// Returns up to max_count of the most recent search terms for the specified
	// keyword.
	void GetMostRecentKeywordSearchTerms(
	KeywordID keyword_id,
	const std::u16string& prefix,
	int max_count,
	std::vector<KeywordSearchTermVisit>* matches);

	// Returns the most recent (i.e., no older than `age_threshold`) normalized
	// search terms (i.e., search terms in lower case with whitespaces collapsed)
	// for the specified keyword.
	std::vector<NormalizedKeywordSearchTermVisit>
	GetMostRecentNormalizedKeywordSearchTerms(KeywordID keyword_id,
	base::Time age_threshold);

	// Deletes all searches matching `term`.
	bool DeleteKeywordSearchTerm(const std::u16string& term);

	// Deletes any search corresponding to `normalized_term`.
	bool DeleteKeywordSearchTermForNormalizedTerm(
	KeywordID keyword_id,
	const std::u16string& normalized_term);

	// Deletes any search corresponding to `url_id`.
	bool DeleteKeywordSearchTermForURL(URLID url_id);

	protected:
	friend class VisitDatabase;

	// See HISTORY_URL_ROW_FIELDS below.
	static const char kURLRowFields[];

	// The number of fiends in kURLRowFields. If callers need additional
	// fields, they can add their 0-based index to this value to get the index of
	// fields following kURLRowFields.
	static const int kNumURLRowFields;

	// Drops the starred_id column from urls, returning true on success. This does
	// nothing (and returns true) if the urls doesn't contain the starred_id
	// column.
	bool DropStarredIDFromURLs();

	// Initialization functions. The indexing functions are separate from the
	// table creation functions so the in-memory database and the temporary tables
	// used when clearing history can populate the table and then create the
	// index, which is faster than the reverse.
	//
	// is_temporary is false when generating the "regular" URLs table. The expirer
	// sets this to true to generate the temporary table, which will have a
	// different name but the same schema.
	bool CreateURLTable(bool is_temporary);

	// Creates the index over URLs so we can quickly look up based on URL.
	bool CreateMainURLIndex();

	// Recreate URL table, and keep all existing contents.
	bool RecreateURLTableWithAllContents();

	// Ensures the keyword search terms table exists.
	bool InitKeywordSearchTermsTable();

	// Creates the indices used for keyword search terms.
	bool CreateKeywordSearchTermsIndices();

	// Deletes the keyword search terms table.
	bool DropKeywordSearchTermsTable();

	// Inserts the given URL row into the URLs table, using the regular table
	// if is_temporary is false, or the temporary URL table if is temporary is
	// true. The current `id` of `info` will be ignored in both cases and a new ID
	// will be generated, which will also constitute the return value, except in
	// case of an error, when the return value is 0. The temporary table may only
	// be used in between CreateTemporaryURLTable() and CommitTemporaryURLTable().
	URLID AddURLInternal(const URLRow& info, bool is_temporary);

	// Return ture if the urls table's schema contains "AUTOINCREMENT".
	// false if table do not contain AUTOINCREMENT, or the table is not created.
	bool URLTableContainsAutoincrement();

	// Convenience to fill a URLRow. Must be in sync with the fields in
	// kHistoryURLRowFields.
	static void FillURLRow(sql::Statement& s, URLRow* i);

	// Returns the database for the functions in this interface. The descendant of
	// this class implements these functions to return its objects.
	virtual sql::Database& GetDB() = 0;

	// Replaces the lower_term column in the keyword search terms table with
	// normalized_term which contains the search term, in lower case, and with
	// whitespaces collapsed for migration to version 42.
	bool MigrateKeywordsSearchTermsLowerTermColumn();

	private:
	// True if InitKeywordSearchTermsTable() has been invoked. Not all subclasses
	// have keyword search terms.
	bool has_keyword_search_terms_;
	};

	// The fields and order expected by FillURLRow(). ID is guaranteed to be first
	// so that DISTINCT can be prepended to get distinct URLs.
	//
	// This is available BOTH as a macro and a static string (kURLRowFields). Use
	// the macro if you want to put this in the middle of an otherwise constant
	// string, it will save time doing string appends. If you have to build a SQL
	// string dynamically anyway, use the constant, it will save space.
	#define HISTORY_URL_ROW_FIELDS \
	" urls.id, urls.url, urls.title, urls.visit_count, urls.typed_count, " \
	"urls.last_visit_time, urls.hidden "

	// Constants which specify, when considered altogether, 'significant'
	// history items. These are used to filter out insignificant items
	// for consideration as autocomplete candidates.
	extern const int kLowQualityMatchTypedLimit;
	extern const int kLowQualityMatchVisitLimit;
	extern const int kLowQualityMatchAgeLimitInDays;

	// The time interval within which a duplicate query is considered invalid for
	// autocomplete purposes.
	// These invalid duplicates are extracted from search query URLs which are
	// identical or nearly identical to the original search query URL and issued too
	// closely to it, i.e., within this time interval. They are typically recorded
	// as a result of back/forward navigations or user interactions in the search
	// result page and are likely not newly initiated searches.
	extern const base::TimeDelta kAutocompleteDuplicateVisitIntervalThreshold;

	// Returns the date threshold for considering an history item as significant.
	base::Time AutocompleteAgeThreshold();

	// Return true if `row` qualifies as an autocomplete candidate. If `threshold`
	// is_null() then this function determines a new time threshold each time it is
	// called. Since getting system time can be costly (such as for cases where
	// this function will be called in a loop over many history items), you can
	// provide a non-null `threshold` by simply initializing `threshold` with
	// AutocompleteAgeThreshold() (or any other desired time in the past).
	bool RowQualifiesAsSignificant(const URLRow& row, const base::Time& threshold);

	} // namespace history

	#endif // COMPONENTS_HISTORY_CORE_BROWSER_URL_DATABASE_H_