This patch implements holdable cursors, following the proposal

(materialization into a tuple store) discussed on pgsql-hackers earlier. I've updated the documentation and the regression tests. Notes on the implementation: - I needed to change the tuple store API slightly -- it assumes that it won't be used to hold data across transaction boundaries, so the temp files that it uses for on-disk storage are automatically reclaimed at end-of-transaction. I added a flag to tuplestore_begin_heap() to control this behavior. Is changing the tuple store API in this fashion OK? - in order to store executor results in a tuple store, I added a new CommandDest. This works well for the most part, with one exception: the current DestFunction API doesn't provide enough information to allow the Executor to store results into an arbitrary tuple store (where the particular tuple store to use is chosen by the call site of ExecutorRun). To workaround this, I've temporarily hacked up a solution that works, but is not ideal: since the receiveTuple DestFunction is passed the portal name, we can use that to lookup the Portal data structure for the cursor and then use that to get at the tuple store the Portal is using. This unnecessarily ties the Portal code with the tupleReceiver code, but it works... The proper fix for this is probably to change the DestFunction API -- Tom suggested passing the full QueryDesc to the receiveTuple function. In that case, callers of ExecutorRun could "subclass" QueryDesc to add any additional fields that their particular CommandDest needed to get access to. This approach would work, but I'd like to think about it for a little bit longer before deciding which route to go. In the mean time, the code works fine, so I don't think a fix is urgent. - (semi-related) I added a NO SCROLL keyword to DECLARE CURSOR, and adjusted the behavior of SCROLL in accordance with the discussion on -hackers. - (unrelated) Cleaned up some SGML markup in sql.sgml, copy.sgml Neil Conway
2025-12-21 05:21:08 +03:00 · 2003-03-27 16:51:29 +00:00
parent 7638087882
commit 54f7338fa1
35 changed files with 850 additions and 314 deletions
--- a/doc/src/sgml/ref/copy.sgml
+++ b/doc/src/sgml/ref/copy.sgml
@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/copy.sgml,v 1.40 2002/09/21 18:32:54 petere Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/copy.sgml,v 1.41 2003/03/27 16:51:27 momjian Exp $
 PostgreSQL documentation
 -->

@@ -133,9 +133,10 @@ COPY <replaceable class="parameter">table</replaceable> [ ( <replaceable class="
       </para>
       <note>
        <para>
-         On a copy in, any data item that matches this string will be stored as
-         a NULL value, so you should make sure that you use the same string
-         as you used on copy out.
+         On a <command>COPY FROM</command>, any data item that matches
+         this string will be stored as a NULL value, so you should
+         make sure that you use the same string as you used with
+         <command>COPY TO</command>.
        </para>
       </note>
      </listitem>
@@ -210,7 +211,8 @@ ERROR: <replaceable>reason</replaceable>
   or write to a file. The file must be accessible to the backend and
   the name must be specified from the viewpoint of the backend. When
   <filename>stdin</filename> or <filename>stdout</filename> is
-   specified, data flows through the client frontend to the backend.
+   specified, data is transmitted via the connection between the
+   client frontend and the backend.
    
    <tip>
     <para>
@@ -234,8 +236,8 @@ ERROR: <replaceable>reason</replaceable>
    Notes
   </title>
   <para>
-   <command>COPY</command> can only be used with plain tables, not with
-   views.
+    <command>COPY</command> can only be used with plain tables, not
+    with views.
   </para>

   <para>
--- a/doc/src/sgml/ref/declare.sgml
+++ b/doc/src/sgml/ref/declare.sgml
@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/declare.sgml,v 1.20 2003/03/21 17:11:46 momjian Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/declare.sgml,v 1.21 2003/03/27 16:51:27 momjian Exp $
 PostgreSQL documentation
 -->

@@ -21,8 +21,8 @@ PostgreSQL documentation
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
-DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INSENSITIVE ] [ SCROLL ]
-    CURSOR FOR <replaceable class="parameter">query</replaceable>
+DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INSENSITIVE ] [ [ NO ] SCROLL ]
+    CURSOR [ { WITH | WITHOUT } HOLD ] FOR <replaceable class="parameter">query</replaceable>
    [ FOR { READ ONLY | UPDATE [ OF <replaceable class="parameter">column</replaceable> [, ...] ] ]
  </synopsis>
  <refsect2 id="R2-SQL-DECLARE-1">
@@ -38,7 +38,8 @@ DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INS
      <term><replaceable class="parameter">cursorname</replaceable></term>
      <listitem>
       <para>
-	The name of the cursor to be used in subsequent FETCH operations.
+	The name of the cursor to be used in subsequent
+	<command>FETCH</command> operations.
       </para>
      </listitem>
     </varlistentry>
@@ -57,8 +58,20 @@ DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INS
      <listitem>
       <para>
 	<acronym>SQL92</acronym> keyword indicating that data retrieved
-	from the cursor should be unaffected by updates from other processes or cursors.
-	By default, all cursors are insensitive.  This keyword has no effect.
+	from the cursor should be unaffected by updates from other
+	processes or cursors.  By default, all cursors are insensitive.
+	This keyword currently has no effect and is present for
+	compatibility with the SQL standard.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term>NO SCROLL</term>
+      <listitem>
+       <para>
+    Specifies that the cursor cannot be used to retrieve rows in a
+    nonsequential fashion (e.g., backward).
       </para>
      </listitem>
     </varlistentry>
@@ -67,8 +80,33 @@ DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INS
      <term>SCROLL</term>
      <listitem>
       <para>
-	Specifies that the cursor may be used to retrieve rows
-	in a nonsequential fashion (e.g., backwards).
+	Specifies that the cursor may be used to retrieve rows in a
+	nonsequential fashion (e.g., backward). Depending upon the
+	complexity of the query's execution plan, specifying
+	<literal>SCROLL</literal> may impose a slight performance penalty
+	on the query's execution time.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term>WITHOUT HOLD</term>
+      <listitem>
+       <para>
+        Specifies that the cursor cannot be used outside of the
+        transaction that created it. If neither <literal>WITHOUT
+        HOLD</literal> nor <literal>WITH HOLD</literal> is specified,
+        <literal>WITH HOLD</literal> is the default.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term>WITH HOLD</term>
+      <listitem>
+       <para>
+        Specifies that the cursor may be used after the transaction
+        that creates it successfully commits.
       </para>
      </listitem>
     </varlistentry>
@@ -124,7 +162,8 @@ DECLARE <replaceable class="parameter">cursorname</replaceable> [ BINARY ] [ INS
   </para>

   <para>
-    The BINARY, INSENSITIVE, and SCROLL keywords may appear in any order.
+    The <literal>BINARY</literal>, <literal>INSENSITIVE</literal>,
+    <literal>SCROLL</literal> keywords may appear in any order.
   </para>
  </refsect2>

@@ -144,7 +183,7 @@ DECLARE CURSOR
       </computeroutput></term>
      <listitem>
       <para>
-	The message returned if the SELECT is run successfully.
+	The message returned if the <command>SELECT</command> is run successfully.
       </para>
      </listitem>
     </varlistentry>
@@ -155,9 +194,8 @@ WARNING:  Closing pre-existing portal "<replaceable class="parameter">cursorname
       </computeroutput></term>
      <listitem>
       <para>
-	This message is reported if the same cursor name was already declared
-	in the current transaction block.  The previous definition is
-	discarded.
+	This message is reported if a cursor with the same name already
+	exists. The previous definition is discarded.
       </para>
      </listitem>
     </varlistentry>
@@ -168,7 +206,9 @@ ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
       </computeroutput></term>
      <listitem>
       <para>
-	This error occurs if the cursor is not declared within a transaction block.
+	This error occurs if the cursor is not declared within a
+	transaction block, and <literal>WITH HOLD</literal> is not
+	specified.
       </para>
      </listitem>
     </varlistentry>     
@@ -193,16 +233,14 @@ ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
  </para>

  <para>
-   Normal cursors return data in text format, the same as a <command>SELECT</>
-   would produce.  Since
-   data is stored natively in binary format, the system must
-   do a conversion to produce the text format. In addition,
-   text formats are often larger in size than the corresponding binary format.
-   Once the information comes back in text form,  the client
-   application may need to convert it to a binary format to
-   manipulate it.
-   BINARY cursors give you back the data in the native binary
-   representation.
+   Normal cursors return data in text format, the same as a
+   <command>SELECT</> would produce.  Since data is stored natively in
+   binary format, the system must do a conversion to produce the text
+   format. In addition, text formats are often larger in size than the
+   corresponding binary format.  Once the information comes back in
+   text form, the client application may need to convert it to a
+   binary format to manipulate it.  BINARY cursors give you back the
+   data in the native binary representation.
  </para>

  <para>
@@ -245,7 +283,9 @@ ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
   </title>

   <para>
-    Cursors are only available within transactions. Use
+    If <literal>WITH HOLD</literal> is not specified, the cursor
+    created by this command can only be used within the current
+    transaction. Use
    <xref linkend="sql-begin" endterm="sql-begin-title">,
    <xref linkend="sql-commit" endterm="sql-commit-title">
    and
@@ -254,12 +294,25 @@ ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
   </para>

   <para>
-    The <literal>SCROLL</> option should be specified when defining a cursor 
-    that will be used to fetch backwards.  This is required by
-    <acronym>SQL92</acronym>.  However, for compatibility with
-    earlier versions, <productname>PostgreSQL</productname> will allow
-    backward fetches without <literal>SCROLL</>, if the cursor's query plan
-    is simple enough that no extra overhead is needed to support it.
+    If <literal>WITH HOLD</literal> is specified and the transaction
+    that created the cursor successfully commits, the cursor can be
+    accessed outside the creating transaction. If the creating
+    transaction is aborted, the cursor is removed. A cursor created
+    with <literal>WITH HOLD</literal> is closed when an explicit
+    <command>CLOSE</command> command is issued on it, or the client
+    connection is terminated.
+   </para>
+
+   <para>
+    The <literal>SCROLL</> option should be specified when defining a
+    cursor that will be used to fetch backwards.  This is required by
+    <acronym>SQL92</acronym>.  However, for compatibility with earlier
+    versions, <productname>PostgreSQL</productname> will allow
+    backward fetches without <literal>SCROLL</>, if the cursor's query
+    plan is simple enough that no extra overhead is needed to support
+    it. However, application developers are advised not to rely on
+    using backward fetches from a cursor that has not been created
+    with <literal>SCROLL</literal>.
   </para>

   <para>
@@ -271,7 +324,7 @@ ERROR:  DECLARE CURSOR may only be used in begin/end transaction blocks
    However, <application>ecpg</application>, the
    embedded SQL preprocessor for <productname>PostgreSQL</productname>,
    supports the <acronym>SQL92</acronym> cursor conventions, including those
-    involving DECLARE and OPEN statements.
+    involving <command>DECLARE</command> and <command>OPEN</command> statements.
   </para>
  </refsect2>
 </refsect1>
@@ -303,13 +356,21 @@ DECLARE liahona CURSOR
    SQL92
   </title>
   <para>
-    <acronym>SQL92</acronym> allows cursors only in embedded <acronym>SQL</acronym>
-    and in modules. <productname>PostgreSQL</productname> permits cursors to be used
-    interactively.
+   <para>
+    <acronym>SQL92</acronym> allows cursors only in embedded
+    <acronym>SQL</acronym> and in modules. <productname>PostgreSQL</>
+    permits cursors to be used interactively.
+   </para>
+
+   <para>
    <acronym>SQL92</acronym> allows embedded or modular cursors to
-    update database information.
-    All <productname>PostgreSQL</productname> cursors are read only.
-    The BINARY keyword is a <productname>PostgreSQL</productname> extension.
+    update database information. All <productname>PostgreSQL</>
+    cursors are read only.
+   </para>
+
+   <para>
+    The <literal>BINARY</literal> keyword is a
+    <productname>PostgreSQL</productname> extension.
   </para>
  </refsect2>
 </refsect1>
--- a/doc/src/sgml/ref/fetch.sgml
+++ b/doc/src/sgml/ref/fetch.sgml
@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/fetch.sgml,v 1.27 2003/03/11 19:40:22 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/fetch.sgml,v 1.28 2003/03/27 16:51:27 momjian Exp $
 PostgreSQL documentation
 -->

@@ -251,8 +251,7 @@ WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</rep
       </computeroutput></term>
      <listitem>
       <para>
-	If <replaceable class="PARAMETER">cursor</replaceable> is not known.
-	The cursor must have been declared within the current transaction block.
+        There is no cursor with the specified name.
       </para>
      </listitem>
     </varlistentry>
@@ -326,7 +325,9 @@ WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</rep
    use any variants of <command>FETCH</> other than <command>FETCH NEXT</>
    or <command>FETCH FORWARD</> with a positive count.  For simple queries
    <productname>PostgreSQL</productname> will allow backwards fetch from
-    cursors not declared with SCROLL, but this behavior is best not relied on.
+    cursors not declared with SCROLL, but this behavior is best not
+    relied on. If the cursor is declared with NO SCROLL, no backward
+    fetches are allowed.
   </para>

   <para>
@@ -339,16 +340,11 @@ WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</rep
   </para>

   <para>
-    Updating data via a cursor is not supported by 
-    <productname>PostgreSQL</productname>,
-    because mapping cursor updates back to base tables is
-    not generally possible, as is also the case with VIEW updates.
-    Consequently,
-    users must issue explicit UPDATE commands to replace data.
-   </para>
-
-   <para>
-    Cursors may only be used inside transaction blocks.
+    Updating data via a cursor is not supported by
+    <productname>PostgreSQL</productname>, because mapping cursor
+    updates back to base tables is not generally possible, as is also
+    the case with view updates.  Consequently, users must issue
+    explicit <command>UPDATE</command> commands to replace data.
   </para>

   <para>
@@ -357,12 +353,6 @@ WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</rep
    Use
    <xref linkend="sql-move" endterm="sql-move-title">
    to change cursor position without retrieving data.
-    Refer to
-    <xref linkend="sql-begin" endterm="sql-begin-title">,
-    <xref linkend="sql-commit" endterm="sql-commit-title">,
-    and
-    <xref linkend="sql-rollback" endterm="sql-rollback-title">
-    for further information about transactions.
   </para>
  </refsect2>
 </refsect1>
@@ -379,7 +369,7 @@ WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</rep
 -- Set up and use a cursor:

 BEGIN WORK;
-DECLARE liahona CURSOR FOR SELECT * FROM films;
+DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films;

 -- Fetch first 5 rows in the cursor liahona:
 FETCH FORWARD 5 IN liahona;
@@ -425,9 +415,10 @@ COMMIT WORK;
   </title>

   <para>
-    <acronym>SQL92</acronym> defines FETCH for use in embedded contexts only.
-    Therefore, it describes placing the results into explicit variables using
-    an <literal>INTO</> clause, for example:
+    <acronym>SQL92</acronym> defines <command>FETCH</command> for use
+    in embedded contexts only.  Therefore, it describes placing the
+    results into explicit variables using an <literal>INTO</> clause,
+    for example:

    <synopsis>
 FETCH ABSOLUTE <replaceable class="PARAMETER">n</replaceable>
@@ -435,16 +426,18 @@ FETCH ABSOLUTE <replaceable class="PARAMETER">n</replaceable>
    INTO :<replaceable class="PARAMETER">variable</replaceable> [, ...]
    </synopsis>

-    <productname>PostgreSQL</productname>'s use of non-embedded cursors
-    is non-standard, and so is its practice of returning the result data
-    as if it were a SELECT result.  Other than this point, FETCH is fully
+    <productname>PostgreSQL</productname>'s use of non-embedded
+    cursors is non-standard, and so is its practice of returning the
+    result data as if it were a <command>SELECT</command> result.
+    Other than this point, <command>FETCH</command> is fully
    upward-compatible with <acronym>SQL92</acronym>.
   </para>

   <para>
-    The FETCH forms involving FORWARD and BACKWARD (including the forms
-    FETCH <replaceable class="PARAMETER">count</replaceable> and FETCH ALL,
-    in which FORWARD is implicit) are <productname>PostgreSQL</productname>
+    The <command>FETCH</command> forms involving FORWARD and BACKWARD
+    (including the forms FETCH <replaceable
+    class="PARAMETER">count</replaceable> and FETCH ALL, in which
+    FORWARD is implicit) are <productname>PostgreSQL</productname>
    extensions.
   </para>

--- a/doc/src/sgml/ref/move.sgml
+++ b/doc/src/sgml/ref/move.sgml
@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/move.sgml,v 1.20 2003/03/11 19:40:22 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/move.sgml,v 1.21 2003/03/27 16:51:27 momjian Exp $
 PostgreSQL documentation
 -->

@@ -64,12 +64,6 @@ MOVE [ <replaceable class="PARAMETER">direction</replaceable> { FROM | IN } ] <r
    Refer to 
    <xref linkend="sql-declare" endterm="sql-declare-title">
    to define a cursor.
-    Refer to 
-    <xref linkend="sql-begin" endterm="sql-begin-title">, 
-    <xref linkend="sql-commit" endterm="sql-commit-title">,
-    and
-    <xref linkend="sql-rollback" endterm="sql-rollback-title">
-    for further information about transactions.
   </para>
  </refsect2>
 </refsect1>
@@ -83,7 +77,7 @@ MOVE [ <replaceable class="PARAMETER">direction</replaceable> { FROM | IN } ] <r

 <programlisting>
 BEGIN WORK;
-DECLARE liahona CURSOR  FOR SELECT * FROM films;
+DECLARE liahona CURSOR FOR SELECT * FROM films;
 -- Skip first 5 rows:
 MOVE FORWARD 5 IN liahona;
 <computeroutput>