datetime.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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$PostgreSQL: pgsql/doc/src/sgml/datetime.sgml,v 2.47 2005/09/09 02:31:48 tgl Exp $
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 <appendix id="datetime-appendix">
  <title>Date/Time Support</title>

  <para>
   <productname>PostgreSQL</productname> uses an internal heuristic
   parser for all date/time input support. Dates and times are input as
   strings, and are broken up into distinct fields with a preliminary
   determination of what kind of information may be in the
   field. Each field is interpreted and either assigned a numeric
   value, ignored, or rejected.
   The parser contains internal lookup tables for all textual fields,
   including months, days of the week, and time zones.
  </para>

  <para>
   This appendix includes information on the content of these
   lookup tables and describes the steps used by the parser to decode
   dates and times.
  </para>

  <sect1>
   <title>Date/Time Input Interpretation</title>

   <para>
    The date/time type inputs are all decoded using the following procedure.
   </para>

   <procedure>
    <step>
     <para>
      Break the input string into tokens and categorize each token as
      a string, time, time zone, or number.
     </para>

     <substeps>
      <step>
       <para>
        If the numeric token contains a colon (<literal>:</>), this is
        a time string. Include all subsequent digits and colons.
       </para>
      </step>

      <step>
       <para>
        If the numeric token contains a dash (<literal>-</>), slash
        (<literal>/</>), or two or more dots (<literal>.</>), this is
        a date string which may have a text month.
       </para>
      </step>

      <step>
       <para>
        If the token is numeric only, then it is either a single field
        or an ISO 8601 concatenated date (e.g.,
        <literal>19990113</literal> for January 13, 1999) or time
        (e.g., <literal>141516</literal> for 14:15:16).
       </para>
      </step>

      <step>
       <para>
        If the token starts with a plus (<literal>+</>) or minus
        (<literal>-</>), then it is either a time zone or a special
        field.
       </para>
      </step>
     </substeps>
    </step>

    <step>
     <para>
      If the token is a text string, match up with possible strings.
     </para>
     
     <substeps>
      <step>
       <para>
        Do a binary-search table lookup for the token
        as either a special string (e.g., <literal>today</literal>),
        day (e.g., <literal>Thursday</literal>),
        month (e.g., <literal>January</literal>),
        or noise word (e.g., <literal>at</literal>, <literal>on</literal>).
       </para>

       <para>
        Set field values and bit mask for fields.
        For example, set year, month, day for <literal>today</literal>,
        and additionally hour, minute, second for <literal>now</literal>.
       </para>
      </step>
      
      <step>
       <para>
        If not found, do a similar binary-search table lookup to match
        the token with a time zone.
       </para>
      </step>

      <step>
       <para>
        If still not found, throw an error.
       </para>
      </step>
     </substeps>
    </step>
    
    <step>
     <para>
      When the token is a number or number field:
     </para>

     <substeps>
      <step>
       <para>
        If there are eight or six digits,
        and if no other date fields have been previously read, then interpret 
        as a <quote>concatenated date</quote> (e.g.,
        <literal>19990118</literal> or <literal>990118</literal>).
        The interpretation is <literal>YYYYMMDD</> or <literal>YYMMDD</>.
       </para>
      </step>

      <step>
       <para>
        If the token is three digits
        and a year has already been read, then interpret as day of year.
       </para>
      </step>
      
      <step>
       <para>
        If four or six digits and a year has already been read, then
        interpret as a time (<literal>HHMM</> or <literal>HHMMSS</>).
       </para>
      </step>

      <step>
       <para>
        If three or more digits and no date fields have yet been found,
        interpret as a year (this forces yy-mm-dd ordering of the remaining
        date fields).
       </para>
      </step>

      <step>
       <para>
        Otherwise the date field ordering is assumed to follow the
        <varname>DateStyle</> setting: mm-dd-yy, dd-mm-yy, or yy-mm-dd.
        Throw an error if a month or day field is found to be out of range.
       </para>
      </step>
     </substeps>
    </step>

    <step>
     <para>
      If BC has been specified, negate the year and add one for
      internal storage.  (There is no year zero in the Gregorian
      calendar, so numerically 1 BC becomes year zero.)
     </para>
    </step>

    <step>
     <para>
      If BC was not specified, and if the year field was two digits in length,
      then adjust the year to four digits. If the field is less than 70, then
      add 2000, otherwise add 1900.

      <tip>
       <para>
        Gregorian years AD 1-99 may be entered by using 4 digits with leading
        zeros (e.g., <literal>0099</> is AD 99). Previous versions of
        <productname>PostgreSQL</productname> accepted years with three
        digits and with single digits, but as of version 7.0 the rules have
        been tightened up to reduce the possibility of ambiguity.
       </para>
      </tip>
     </para>
    </step>
   </procedure>
  </sect1>


  <sect1 id="datetime-keywords">
   <title>Date/Time Key Words</title>

   <para>
    <xref linkend="datetime-month-table"> shows the tokens that are
    recognized as names of months.
   </para>

    <table id="datetime-month-table">
     <title>Month Names</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Month</entry>
        <entry>Abbreviations</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>January</entry>
        <entry>Jan</entry>
       </row>
       <row>
        <entry>February</entry>
        <entry>Feb</entry>
       </row>
       <row>
        <entry>March</entry>
        <entry>Mar</entry>
       </row>
       <row>
        <entry>April</entry>
        <entry>Apr</entry>
       </row>
       <row>
        <entry>May</entry>
        <entry></entry>
       </row>
       <row>
        <entry>June</entry>
        <entry>Jun</entry>
       </row>
       <row>
        <entry>July</entry>
        <entry>Jul</entry>
       </row>
       <row>
        <entry>August</entry>
        <entry>Aug</entry>
       </row>
       <row>
        <entry>September</entry>
        <entry>Sep, Sept</entry>
       </row>
       <row>
        <entry>October</entry>
        <entry>Oct</entry>
       </row>
       <row>
        <entry>November</entry>
        <entry>Nov</entry>
       </row>
       <row>
        <entry>December</entry>
        <entry>Dec</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
     <xref linkend="datetime-dow-table"> shows the tokens that are
     recognized as names of days of the week.
    </para>

     <table id="datetime-dow-table">
      <title>Day of the Week Names</title>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Day</entry>
         <entry>Abbreviations</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>Sunday</entry>
         <entry>Sun</entry>
        </row>
        <row>
         <entry>Monday</entry>
         <entry>Mon</entry>
        </row>
        <row>
         <entry>Tuesday</entry>
         <entry>Tue, Tues</entry>
        </row>
        <row>
         <entry>Wednesday</entry>
         <entry>Wed, Weds</entry>
        </row>
        <row>
         <entry>Thursday</entry>
         <entry>Thu, Thur, Thurs</entry>
        </row>
        <row>
         <entry>Friday</entry>
         <entry>Fri</entry>
        </row>
        <row>
         <entry>Saturday</entry>
         <entry>Sat</entry>
        </row>
       </tbody>
      </tgroup>
     </table>

   <para>
    <xref linkend="datetime-mod-table"> shows the tokens that serve
    various modifier purposes.
   </para>

    <table id="datetime-mod-table">
     <title>Date/Time Field Modifiers</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Identifier</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>ABSTIME</literal></entry>
        <entry>Ignored</entry>
       </row>
       <row>
        <entry><literal>AM</literal></entry>
        <entry>Time is before 12:00</entry>
       </row>
       <row>
        <entry><literal>AT</literal></entry>
        <entry>Ignored</entry>
       </row>
       <row>
        <entry><literal>JULIAN</>, <literal>JD</>, <literal>J</></entry>
        <entry>Next field is Julian Day</entry>
       </row>
       <row>
        <entry><literal>ON</literal></entry>
        <entry>Ignored</entry>
       </row>
       <row>
        <entry><literal>PM</literal></entry>
        <entry>Time is on or after 12:00</entry>
       </row>
       <row>
        <entry><literal>T</literal></entry>
        <entry>Next field is time</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The key word <literal>ABSTIME</literal> is ignored for historical
    reasons: In very old releases of
    <productname>PostgreSQL</productname>, invalid values of type <type>abstime</type>
    were emitted as <literal>Invalid Abstime</literal>. This is no
    longer the case however and this key word will likely be dropped in
    a future release.
   </para>

   <indexterm>
    <primary>time zone</primary>
    <secondary>input abbreviations</secondary>
   </indexterm>

   <para>
    <xref linkend="datetime-timezone-input-table"> shows the time zone
    abbreviations recognized by <productname>PostgreSQL</productname>
    in date/time input values.  Note that these names are <emphasis>not</>
    necessarily used for date/time output &mdash; output is driven by the
    official timezone abbreviation(s) associated with the currently selected
    <xref linkend="guc-timezone"> parameter setting.  (It is
    likely that future releases will make some use of <varname>timezone</>
    for input as well.)
   </para>

   <para>
    The table is organized by time zone offset from <acronym>UTC</>,
    rather than alphabetically.  This is intended to facilitate
    matching local usage with recognized abbreviations for cases where
    these might differ.
   </para>

    <table id="datetime-timezone-input-table">
     <title>Time Zone Abbreviations for Input</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Time Zone</entry>
        <entry>Offset from UTC</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>NZDT</entry>
        <entry>+13:00</entry>
        <entry>New Zealand Daylight-Saving Time</entry>
       </row>
       <row>
        <entry>IDLE</entry>
        <entry>+12:00</entry>
        <entry>International Date Line, East</entry>
       </row>
       <row>
        <entry>NZST</entry>
        <entry>+12:00</entry>
        <entry>New Zealand Standard Time</entry>
       </row>
       <row>
        <entry>NZT</entry>
        <entry>+12:00</entry>
        <entry>New Zealand Time</entry>
       </row>
       <row>
        <entry>AESST</entry>
        <entry>+11:00</entry>
        <entry>Australia Eastern Summer Standard Time</entry>
       </row>
       <row>
        <entry>ACSST</entry>
        <entry>+10:30</entry>
        <entry>Central Australia Summer Standard Time</entry>
       </row>
       <row>
        <entry>CADT</entry>
        <entry>+10:30</entry>
        <entry>Central Australia Daylight-Saving Time</entry>
       </row>
       <row>
        <entry>SADT</entry>
        <entry>+10:30</entry>
        <entry>South Australian Daylight-Saving Time</entry>
       </row>
       <row>
        <entry>AEST</entry>
        <entry>+10:00</entry>
        <entry>Australia Eastern Standard Time</entry>
       </row>
       <row>
        <entry>EAST</entry>
        <entry>+10:00</entry>
        <entry>East Australian Standard Time</entry>
       </row>
       <row>
        <entry>GST</entry>
        <entry>+10:00</entry>
        <entry>Guam Standard Time, Russia zone 9</entry>
       </row>
       <row>
        <entry>LIGT</entry>
        <entry>+10:00</entry>
        <entry>Melbourne, Australia</entry>
       </row>
       <row>
        <entry>SAST</entry>
        <entry>+09:30</entry>
        <entry>South Australia Standard Time</entry>
       </row>
       <row>
        <entry>CAST</entry>
        <entry>+09:30</entry>
        <entry>Central Australia Standard Time</entry>
       </row>
       <row>
        <entry>AWSST</entry>
        <entry>+09:00</entry>
        <entry>Australia Western Summer Standard Time</entry>
       </row>
       <row>
        <entry>JST</entry>
        <entry>+09:00</entry>
        <entry>Japan Standard Time, Russia zone 8</entry>
       </row>
       <row>
        <entry>KST</entry>
        <entry>+09:00</entry>
        <entry>Korea Standard Time</entry>
       </row>
       <row>
        <entry>MHT</entry>
        <entry>+09:00</entry>
        <entry>Kwajalein Time</entry>
       </row>
       <row>
        <entry>WDT</entry>
        <entry>+09:00</entry>
        <entry>West Australian Daylight-Saving Time</entry>
       </row>
       <row>
        <entry>MT</entry>
        <entry>+08:30</entry>
        <entry>Moluccas Time</entry>
       </row>
       <row>
        <entry>AWST</entry>
        <entry>+08:00</entry>
        <entry>Australia Western Standard Time</entry>
       </row>
       <row>
        <entry>CCT</entry>
        <entry>+08:00</entry>
        <entry>China Coastal Time</entry>
       </row>
       <row>
        <entry>WADT</entry>
        <entry>+08:00</entry>
        <entry>West Australian Daylight-Saving Time</entry>
       </row>
       <row>
        <entry>WST</entry>
        <entry>+08:00</entry>
        <entry>West Australian Standard Time</entry>
       </row>
       <row>
        <entry>JT</entry>
        <entry>+07:30</entry>
        <entry>Java Time</entry>
       </row>
       <row>
        <entry>ALMST</entry>
        <entry>+07:00</entry>
        <entry>Almaty Summer Time</entry>
       </row>
       <row>
        <entry>WAST</entry>
        <entry>+07:00</entry>
        <entry>West Australian Standard Time</entry>
       </row>
       <row>
        <entry>CXT</entry>
        <entry>+07:00</entry>
        <entry>Christmas (Island) Time</entry>
       </row>
       <row>
        <entry>MMT</entry>
        <entry>+06:30</entry>
        <entry>Myanmar Time</entry>
       </row>
       <row>
        <entry>ALMT</entry>
        <entry>+06:00</entry>
        <entry>Almaty Time</entry>
       </row>
       <row>
        <entry>MAWT</entry>
        <entry>+06:00</entry>
        <entry>Mawson (Antarctica) Time</entry>
       </row>
       <row>
        <entry>IOT</entry>
        <entry>+05:00</entry>
        <entry>Indian Chagos Time</entry>
       </row>
       <row>
        <entry>MVT</entry>
        <entry>+05:00</entry>
        <entry>Maldives Island Time</entry>
       </row>
       <row>
        <entry>TFT</entry>
        <entry>+05:00</entry>
        <entry>Kerguelen Time</entry>
       </row>
       <row>
        <entry>AFT</entry>
        <entry>+04:30</entry>
        <entry>Afghanistan Time</entry>
       </row>
       <row>
        <entry>EAST</entry>
        <entry>+04:00</entry>
        <entry>Antananarivo Summer Time</entry>
       </row>
       <row>
        <entry>MUT</entry>
        <entry>+04:00</entry>
        <entry>Mauritius Island Time</entry>
       </row>
       <row>
        <entry>RET</entry>
        <entry>+04:00</entry>
        <entry>Reunion Island Time</entry>
       </row>
       <row>
        <entry>SCT</entry>
        <entry>+04:00</entry>
        <entry>Mahe Island Time</entry>
       </row>
       <row>
        <entry>IRT, IT</entry>
        <entry>+03:30</entry>
        <entry>Iran Time</entry>
       </row>
       <row>