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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/ecpg-descriptors.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>35.7. Using Descriptor Areas</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="ecpg-pgtypes.html" title="35.6. pgtypes Library" /><link rel="next" href="ecpg-errors.html" title="35.8. Error Handling" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">35.7. Using Descriptor Areas</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-pgtypes.html" title="35.6. pgtypes Library">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 35. ECPG - Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 35. <span xmlns="http://www.w3.org/1999/xhtml" class="application">ECPG</span> - Embedded <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-errors.html" title="35.8. Error Handling">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="ECPG-DESCRIPTORS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">35.7. Using Descriptor Areas</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-descriptors.html#ECPG-NAMED-DESCRIPTORS">35.7.1. Named SQL Descriptor Areas</a></span></dt><dt><span class="sect2"><a href="ecpg-descriptors.html#ECPG-SQLDA-DESCRIPTORS">35.7.2. SQLDA Descriptor Areas</a></span></dt></dl></div><p>

  3.    An SQL descriptor area is a more sophisticated method for processing

  4.    the result of a <code class="command">SELECT</code>, <code class="command">FETCH</code> or

  5.    a <code class="command">DESCRIBE</code> statement. An SQL descriptor area groups

  6.    the data of one row of data together with metadata items into one

  7.    data structure.  The metadata is particularly useful when executing

  8.    dynamic SQL statements, where the nature of the result columns might

  9.    not be known ahead of time. PostgreSQL provides two ways to use

  10.    Descriptor Areas: the named SQL Descriptor Areas and the C-structure

  11.    SQLDAs.

  12.   </p><div class="sect2" id="ECPG-NAMED-DESCRIPTORS"><div class="titlepage"><div><div><h3 class="title">35.7.1. Named SQL Descriptor Areas</h3></div></div></div><p>

  13.     A named SQL descriptor area consists of a header, which contains

  14.     information concerning the entire descriptor, and one or more item

  15.     descriptor areas, which basically each describe one column in the

  16.     result row.

  17.    </p><p>

  18.     Before you can use an SQL descriptor area, you need to allocate one:

  19. </p><pre class="programlisting">

  20. EXEC SQL ALLOCATE DESCRIPTOR <em class="replaceable"><code>identifier</code></em>;

  21. </pre><p>

  22.     The identifier serves as the <span class="quote">“<span class="quote">variable name</span>”</span> of the

  23.     descriptor area.  

  24.     When you don't need the descriptor anymore, you should deallocate

  25.     it:

  26. </p><pre class="programlisting">

  27. EXEC SQL DEALLOCATE DESCRIPTOR <em class="replaceable"><code>identifier</code></em>;

  28. </pre><p>

  29.    </p><p>

  30.     To use a descriptor area, specify it as the storage target in an

  31.     <code class="literal">INTO</code> clause, instead of listing host variables:

  32. </p><pre class="programlisting">

  33. EXEC SQL FETCH NEXT FROM mycursor INTO SQL DESCRIPTOR mydesc;

  34. </pre><p>

  35.     If the result set is empty, the Descriptor Area will still contain

  36.     the metadata from the query, i.e. the field names.

  37.    </p><p>

  38.     For not yet executed prepared queries, the <code class="command">DESCRIBE</code>

  39.     statement can be used to get the metadata of the result set:

  40. </p><pre class="programlisting">

  41. EXEC SQL BEGIN DECLARE SECTION;

  42. char *sql_stmt = "SELECT * FROM table1";

  43. EXEC SQL END DECLARE SECTION;

  44. 

  45. EXEC SQL PREPARE stmt1 FROM :sql_stmt;

  46. EXEC SQL DESCRIBE stmt1 INTO SQL DESCRIPTOR mydesc;

  47. </pre><p>

  48.    </p><p>

  49.     Before PostgreSQL 9.0, the <code class="literal">SQL</code> keyword was optional,

  50.     so using <code class="literal">DESCRIPTOR</code> and <code class="literal">SQL DESCRIPTOR</code>

  51.     produced named SQL Descriptor Areas. Now it is mandatory, omitting

  52.     the <code class="literal">SQL</code> keyword produces SQLDA Descriptor Areas,

  53.     see <a class="xref" href="ecpg-descriptors.html#ECPG-SQLDA-DESCRIPTORS" title="35.7.2. SQLDA Descriptor Areas">Section 35.7.2</a>.

  54.    </p><p>

  55.     In <code class="command">DESCRIBE</code> and <code class="command">FETCH</code> statements,

  56.     the <code class="literal">INTO</code> and <code class="literal">USING</code> keywords can be

  57.     used to similarly: they produce the result set and the metadata in a

  58.     Descriptor Area.

  59.    </p><p>

  60.     Now how do you get the data out of the descriptor area?  You can

  61.     think of the descriptor area as a structure with named fields.  To

  62.     retrieve the value of a field from the header and store it into a

  63.     host variable, use the following command:

  64. </p><pre class="programlisting">

  65. EXEC SQL GET DESCRIPTOR <em class="replaceable"><code>name</code></em> :<em class="replaceable"><code>hostvar</code></em> = <em class="replaceable"><code>field</code></em>;

  66. </pre><p>

  67.     Currently, there is only one header field defined:

  68.     <em class="replaceable"><code>COUNT</code></em>, which tells how many item

  69.     descriptor areas exist (that is, how many columns are contained in

  70.     the result).  The host variable needs to be of an integer type.  To

  71.     get a field from the item descriptor area, use the following

  72.     command:

  73. </p><pre class="programlisting">

  74. EXEC SQL GET DESCRIPTOR <em class="replaceable"><code>name</code></em> VALUE <em class="replaceable"><code>num</code></em> :<em class="replaceable"><code>hostvar</code></em> = <em class="replaceable"><code>field</code></em>;

  75. </pre><p>

  76.     <em class="replaceable"><code>num</code></em> can be a literal integer or a host

  77.     variable containing an integer. Possible fields are:

  78. 

  79.     </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">CARDINALITY</code> (integer)</span></dt><dd><p>

  80.         number of rows in the result set

  81.        </p></dd><dt><span class="term"><code class="literal">DATA</code></span></dt><dd><p>

  82.         actual data item (therefore, the data type of this field

  83.         depends on the query)

  84.        </p></dd><dt><span class="term"><code class="literal">DATETIME_INTERVAL_CODE</code> (integer)</span></dt><dd><p>

  85.         When <code class="literal">TYPE</code> is <code class="literal">9</code>,

  86.         <code class="literal">DATETIME_INTERVAL_CODE</code> will have a value of

  87.         <code class="literal">1</code> for <code class="literal">DATE</code>,

  88.         <code class="literal">2</code> for <code class="literal">TIME</code>,

  89.         <code class="literal">3</code> for <code class="literal">TIMESTAMP</code>,

  90.         <code class="literal">4</code> for <code class="literal">TIME WITH TIME ZONE</code>, or

  91.         <code class="literal">5</code> for <code class="literal">TIMESTAMP WITH TIME ZONE</code>.

  92.        </p></dd><dt><span class="term"><code class="literal">DATETIME_INTERVAL_PRECISION</code> (integer)</span></dt><dd><p>

  93.         not implemented

  94.        </p></dd><dt><span class="term"><code class="literal">INDICATOR</code> (integer)</span></dt><dd><p>

  95.         the indicator (indicating a null value or a value truncation)

  96.        </p></dd><dt><span class="term"><code class="literal">KEY_MEMBER</code> (integer)</span></dt><dd><p>

  97.         not implemented

  98.        </p></dd><dt><span class="term"><code class="literal">LENGTH</code> (integer)</span></dt><dd><p>

  99.         length of the datum in characters

  100.        </p></dd><dt><span class="term"><code class="literal">NAME</code> (string)</span></dt><dd><p>

  101.         name of the column

  102.        </p></dd><dt><span class="term"><code class="literal">NULLABLE</code> (integer)</span></dt><dd><p>

  103.         not implemented

  104.        </p></dd><dt><span class="term"><code class="literal">OCTET_LENGTH</code> (integer)</span></dt><dd><p>

  105.         length of the character representation of the datum in bytes

  106.        </p></dd><dt><span class="term"><code class="literal">PRECISION</code> (integer)</span></dt><dd><p>

  107.         precision (for type <code class="type">numeric</code>)

  108.        </p></dd><dt><span class="term"><code class="literal">RETURNED_LENGTH</code> (integer)</span></dt><dd><p>

  109.         length of the datum in characters

  110.        </p></dd><dt><span class="term"><code class="literal">RETURNED_OCTET_LENGTH</code> (integer)</span></dt><dd><p>

  111.         length of the character representation of the datum in bytes

  112.        </p></dd><dt><span class="term"><code class="literal">SCALE</code> (integer)</span></dt><dd><p>

  113.         scale (for type <code class="type">numeric</code>)

  114.        </p></dd><dt><span class="term"><code class="literal">TYPE</code> (integer)</span></dt><dd><p>

  115.         numeric code of the data type of the column

  116.        </p></dd></dl></div><p>

  117.    </p><p>

  118.     In <code class="command">EXECUTE</code>, <code class="command">DECLARE</code> and <code class="command">OPEN</code>

  119.     statements, the effect of the <code class="literal">INTO</code> and <code class="literal">USING</code>

  120.     keywords are different. A Descriptor Area can also be manually built to

  121.     provide the input parameters for a query or a cursor and

  122.     <code class="literal">USING SQL DESCRIPTOR <em class="replaceable"><code>name</code></em></code>

  123.     is the way to pass the input parameters into a parameterized query. The statement

  124.     to build a named SQL Descriptor Area is below:

  125. </p><pre class="programlisting">

  126. EXEC SQL SET DESCRIPTOR <em class="replaceable"><code>name</code></em> VALUE <em class="replaceable"><code>num</code></em> <em class="replaceable"><code>field</code></em> = :<em class="replaceable"><code>hostvar</code></em>;

  127. </pre><p>

  128.    </p><p>

  129.     PostgreSQL supports retrieving more that one record in one <code class="command">FETCH</code>

  130.     statement and storing the data in host variables in this case assumes that the

  131.     variable is an array. E.g.:

  132. </p><pre class="programlisting">

  133. EXEC SQL BEGIN DECLARE SECTION;

  134. int id[5];

  135. EXEC SQL END DECLARE SECTION;

  136. 

  137. EXEC SQL FETCH 5 FROM mycursor INTO SQL DESCRIPTOR mydesc;

  138. 

  139. EXEC SQL GET DESCRIPTOR mydesc VALUE 1 :id = DATA;

  140. </pre><p>

  141. 

  142.    </p></div><div class="sect2" id="ECPG-SQLDA-DESCRIPTORS"><div class="titlepage"><div><div><h3 class="title">35.7.2. SQLDA Descriptor Areas</h3></div></div></div><p>

  143.     An SQLDA Descriptor Area is a C language structure which can be also used

  144.     to get the result set and the metadata of a query. One structure stores one

  145.     record from the result set.

  146. </p><pre class="programlisting">

  147. EXEC SQL include sqlda.h;

  148. sqlda_t         *mysqlda;

  149. 

  150. EXEC SQL FETCH 3 FROM mycursor INTO DESCRIPTOR mysqlda;

  151. </pre><p>

  152.     Note that the <code class="literal">SQL</code> keyword is omitted. The paragraphs about

  153.     the use cases of the <code class="literal">INTO</code> and <code class="literal">USING</code>

  154.     keywords in <a class="xref" href="ecpg-descriptors.html#ECPG-NAMED-DESCRIPTORS" title="35.7.1. Named SQL Descriptor Areas">Section 35.7.1</a> also apply here with an addition.

  155.     In a <code class="command">DESCRIBE</code> statement the <code class="literal">DESCRIPTOR</code>

  156.     keyword can be completely omitted if the <code class="literal">INTO</code> keyword is used:

  157. </p><pre class="programlisting">

  158. EXEC SQL DESCRIBE prepared_statement INTO mysqlda;

  159. </pre><p>

  160.    </p><div class="procedure"><p>

  161.       The general flow of a program that uses SQLDA is:

  162.      </p><ol class="procedure" type="1"><li class="step"><p>Prepare a query, and declare a cursor for it.</p></li><li class="step"><p>Declare an SQLDA for the result rows.</p></li><li class="step"><p>Declare an SQLDA for the input parameters, and initialize them (memory allocation, parameter settings).</p></li><li class="step"><p>Open a cursor with the input SQLDA.</p></li><li class="step"><p>Fetch rows from the cursor, and store them into an output SQLDA.</p></li><li class="step"><p>Read values from the output SQLDA into the host variables (with conversion if necessary).</p></li><li class="step"><p>Close the cursor.</p></li><li class="step"><p>Free the memory area allocated for the input SQLDA.</p></li></ol></div><div class="sect3" id="id-1.7.5.13.4.4"><div class="titlepage"><div><div><h4 class="title">35.7.2.1. SQLDA Data Structure</h4></div></div></div><p>

  163.      SQLDA uses three data structure

  164.      types: <code class="type">sqlda_t</code>, <code class="type">sqlvar_t</code>,

  165.      and <code class="type">struct sqlname</code>.

  166.     </p><div class="tip"><h3 class="title">Tip</h3><p>

  167.       PostgreSQL's SQLDA has a similar data structure to the one in

  168.       IBM DB2 Universal Database, so some technical information on

  169.       DB2's SQLDA could help understanding PostgreSQL's one better.

  170.      </p></div><div class="sect4" id="ECPG-SQLDA-SQLDA"><div class="titlepage"><div><div><h5 class="title">35.7.2.1.1. sqlda_t Structure</h5></div></div></div><p>

  171.       The structure type <code class="type">sqlda_t</code> is the type of the

  172.       actual SQLDA.  It holds one record.  And two or

  173.       more <code class="type">sqlda_t</code> structures can be connected in a

  174.       linked list with the pointer in

  175.       the <code class="structfield">desc_next</code> field, thus

  176.       representing an ordered collection of rows.  So, when two or

  177.       more rows are fetched, the application can read them by

  178.       following the <code class="structfield">desc_next</code> pointer in

  179.       each <code class="type">sqlda_t</code> node.

  180.      </p><p>

  181.       The definition of <code class="type">sqlda_t</code> is:

  182. </p><pre class="programlisting">

  183. struct sqlda_struct

  184. {

  185.     char            sqldaid[8];

  186.     long            sqldabc;

  187.     short           sqln;

  188.     short           sqld;

  189.     struct sqlda_struct *desc_next;

  190.     struct sqlvar_struct sqlvar[1];

  191. };

  192. 

  193. typedef struct sqlda_struct sqlda_t;

  194. </pre><p>

  195. 

  196.       The meaning of the fields is:

  197. 

  198.     </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">sqldaid</code></span></dt><dd><p>

  199.         It contains the literal string <code class="literal">"SQLDA  "</code>.

  200.        </p></dd><dt><span class="term"><code class="literal">sqldabc</code></span></dt><dd><p>

  201.         It contains the size of the allocated space in bytes.

  202.        </p></dd><dt><span class="term"><code class="literal">sqln</code></span></dt><dd><p>

  203.         It contains the number of input parameters for a parameterized query in

  204.         case it's passed into <code class="command">OPEN</code>, <code class="command">DECLARE</code> or

  205.         <code class="command">EXECUTE</code> statements using the <code class="literal">USING</code>

  206.         keyword. In case it's used as output of <code class="command">SELECT</code>,

  207.         <code class="command">EXECUTE</code> or <code class="command">FETCH</code> statements,

  208.         its value is the same as <code class="literal">sqld</code>

  209.         statement

  210.        </p></dd><dt><span class="term"><code class="literal">sqld</code></span></dt><dd><p>

  211.         It contains the number of fields in a result set.

  212.        </p></dd><dt><span class="term"><code class="literal">desc_next</code></span></dt><dd><p>

  213.         If the query returns more than one record, multiple linked

  214.         SQLDA structures are returned, and <code class="literal">desc_next</code> holds

  215.         a pointer to the next entry in the list.

  216.        </p></dd><dt><span class="term"><code class="literal">sqlvar</code></span></dt><dd><p>

  217.         This is the array of the columns in the result set.

  218.        </p></dd></dl></div><p>

  219.      </p></div><div class="sect4" id="ECPG-SQLDA-SQLVAR"><div class="titlepage"><div><div><h5 class="title">35.7.2.1.2. sqlvar_t Structure</h5></div></div></div><p>

  220.       The structure type <code class="type">sqlvar_t</code> holds a column value

  221.       and metadata such as type and length. The definition of the type

  222.       is:

  223. 

  224. </p><pre class="programlisting">

  225. struct sqlvar_struct

  226. {

  227.     short          sqltype;

  228.     short          sqllen;

  229.     char          *sqldata;

  230.     short         *sqlind;

  231.     struct sqlname sqlname;

  232. };

  233. 

  234. typedef struct sqlvar_struct sqlvar_t;

  235. </pre><p>

  236. 

  237.       The meaning of the fields is:

  238. 

  239.         </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">sqltype</code></span></dt><dd><p>

  240.             Contains the type identifier of the field. For values,

  241.             see <code class="literal">enum ECPGttype</code> in <code class="literal">ecpgtype.h</code>.

  242.            </p></dd><dt><span class="term"><code class="literal">sqllen</code></span></dt><dd><p>

  243.             Contains the binary length of the field. e.g. 4 bytes for <code class="type">ECPGt_int</code>.

  244.            </p></dd><dt><span class="term"><code class="literal">sqldata</code></span></dt><dd><p>

  245.             Points to the data.  The format of the data is described

  246.             in <a class="xref" href="ecpg-variables.html#ECPG-VARIABLES-TYPE-MAPPING" title="35.4.4. Type Mapping">Section 35.4.4</a>.

  247.            </p></dd><dt><span class="term"><code class="literal">sqlind</code></span></dt><dd><p>

  248.             Points to the null indicator.  0 means not null, -1 means

  249.             null.

  250.            </p></dd><dt><span class="term"><code class="literal">sqlname</code></span></dt><dd><p>

  251.             The name of the field.

  252.            </p></dd></dl></div><p>

  253.      </p></div><div class="sect4" id="ECPG-SQLDA-SQLNAME"><div class="titlepage"><div><div><h5 class="title">35.7.2.1.3. struct sqlname Structure</h5></div></div></div><p>

  254.       A <code class="type">struct sqlname</code> structure holds a column name.  It

  255.       is used as a member of the <code class="type">sqlvar_t</code> structure.  The

  256.       definition of the structure is:

  257. </p><pre class="programlisting">

  258. #define NAMEDATALEN 64

  259. 

  260. struct sqlname

  261. {

  262.         short           length;

  263.         char            data[NAMEDATALEN];

  264. };

  265. </pre><p>

  266.       The meaning of the fields is:

  267.             </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">length</code></span></dt><dd><p>

  268.                  Contains the length of the field name.

  269.                 </p></dd><dt><span class="term"><code class="literal">data</code></span></dt><dd><p>

  270.                  Contains the actual field name.

  271.                 </p></dd></dl></div><p>

  272.      </p></div></div><div class="sect3" id="ECPG-SQLDA-OUTPUT"><div class="titlepage"><div><div><h4 class="title">35.7.2.2. Retrieving a Result Set Using an SQLDA</h4></div></div></div><div class="procedure"><p>

  273.       The general steps to retrieve a query result set through an

  274.       SQLDA are:

  275.      </p><ol class="procedure" type="1"><li class="step"><p>Declare an <code class="type">sqlda_t</code> structure to receive the result set.</p></li><li class="step"><p>Execute <code class="command">FETCH</code>/<code class="command">EXECUTE</code>/<code class="command">DESCRIBE</code> commands to process a query specifying the declared SQLDA.</p></li><li class="step"><p>Check the number of records in the result set by looking at <code class="structfield">sqln</code>, a member of the <code class="type">sqlda_t</code> structure.</p></li><li class="step"><p>Get the values of each column from <code class="literal">sqlvar[0]</code>, <code class="literal">sqlvar[1]</code>, etc., members of the <code class="type">sqlda_t</code> structure.</p></li><li class="step"><p>Go to next row (<code class="type">sqlda_t</code> structure) by following the <code class="structfield">desc_next</code> pointer, a member of the <code class="type">sqlda_t</code> structure.</p></li><li class="step"><p>Repeat above as you need.</p></li></ol></div><p>

  276.      Here is an example retrieving a result set through an SQLDA.

  277.     </p><p>

  278.      First, declare a <code class="type">sqlda_t</code> structure to receive the result set.

  279. </p><pre class="programlisting">

  280. sqlda_t *sqlda1;

  281. </pre><p>

  282.     </p><p>

  283.      Next, specify the SQLDA in a command.  This is

  284.      a <code class="command">FETCH</code> command example.

  285. </p><pre class="programlisting">

  286. EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;

  287. </pre><p>

  288.     </p><p>

  289.      Run a loop following the linked list to retrieve the rows.

  290. </p><pre class="programlisting">

  291. sqlda_t *cur_sqlda;

  292. 

  293. for (cur_sqlda = sqlda1;

  294.      cur_sqlda != NULL;

  295.      cur_sqlda = cur_sqlda-&gt;desc_next)

  296. {

  297.     ...

  298. }

  299. </pre><p>

  300.     </p><p>

  301.      Inside the loop, run another loop to retrieve each column data

  302.      (<code class="type">sqlvar_t</code> structure) of the row.

  303. </p><pre class="programlisting">

  304. for (i = 0; i &lt; cur_sqlda-&gt;sqld; i++)

  305. {

  306.     sqlvar_t v = cur_sqlda-&gt;sqlvar[i];

  307.     char *sqldata = v.sqldata;

  308.     short sqllen  = v.sqllen;

  309.     ...

  310. }

  311. </pre><p>

  312.     </p><p>

  313.      To get a column value, check the <code class="structfield">sqltype</code> value,

  314.      a member of the <code class="type">sqlvar_t</code> structure.  Then, switch

  315.      to an appropriate way, depending on the column type, to copy

  316.      data from the <code class="structfield">sqlvar</code> field to a host variable.

  317. </p><pre class="programlisting">

  318. char var_buf[1024];

  319. 

  320. switch (v.sqltype)

  321. {

  322.     case ECPGt_char:

  323.         memset(&amp;var_buf, 0, sizeof(var_buf));

  324.         memcpy(&amp;var_buf, sqldata, (sizeof(var_buf) &lt;= sqllen ? sizeof(var_buf) - 1 : sqllen));

  325.         break;

  326. 

  327.     case ECPGt_int: /* integer */

  328.         memcpy(&amp;intval, sqldata, sqllen);

  329.         snprintf(var_buf, sizeof(var_buf), "%d", intval);

  330.         break;

  331. 

  332.     ...

  333. }

  334. </pre><p>

  335.     </p></div><div class="sect3" id="ECPG-SQLDA-INPUT"><div class="titlepage"><div><div><h4 class="title">35.7.2.3. Passing Query Parameters Using an SQLDA</h4></div></div></div><div class="procedure"><p>

  336.       The general steps to use an SQLDA to pass input

  337.       parameters to a prepared query are:

  338.      </p><ol class="procedure" type="1"><li class="step"><p>Create a prepared query (prepared statement)</p></li><li class="step"><p>Declare a sqlda_t structure as an input SQLDA.</p></li><li class="step"><p>Allocate memory area (as sqlda_t structure) for the input SQLDA.</p></li><li class="step"><p>Set (copy) input values in the allocated memory.</p></li><li class="step"><p>Open a cursor with specifying the input SQLDA.</p></li></ol></div><p>

  339.      Here is an example.

  340.     </p><p>

  341.      First, create a prepared statement.

  342. </p><pre class="programlisting">

  343. EXEC SQL BEGIN DECLARE SECTION;

  344. char query[1024] = "SELECT d.oid, * FROM pg_database d, pg_stat_database s WHERE d.oid = s.datid AND (d.datname = ? OR d.oid = ?)";

  345. EXEC SQL END DECLARE SECTION;

  346. 

  347. EXEC SQL PREPARE stmt1 FROM :query;

  348. </pre><p>

  349.     </p><p>

  350.      Next, allocate memory for an SQLDA, and set the number of input

  351.      parameters in <code class="structfield">sqln</code>, a member variable of

  352.      the <code class="type">sqlda_t</code> structure.  When two or more input

  353.      parameters are required for the prepared query, the application

  354.      has to allocate additional memory space which is calculated by

  355.      (nr. of params - 1) * sizeof(sqlvar_t).  The example shown here

  356.      allocates memory space for two input parameters.

  357. </p><pre class="programlisting">

  358. sqlda_t *sqlda2;

  359. 

  360. sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));

  361. memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));

  362. 

  363. sqlda2-&gt;sqln = 2; /* number of input variables */

  364. </pre><p>

  365.     </p><p>

  366.      After memory allocation, store the parameter values into the

  367.      <code class="literal">sqlvar[]</code> array.  (This is same array used for

  368.      retrieving column values when the SQLDA is receiving a result

  369.      set.)  In this example, the input parameters

  370.      are <code class="literal">"postgres"</code>, having a string type,

  371.      and <code class="literal">1</code>, having an integer type.

  372. </p><pre class="programlisting">

  373. sqlda2-&gt;sqlvar[0].sqltype = ECPGt_char;

  374. sqlda2-&gt;sqlvar[0].sqldata = "postgres";

  375. sqlda2-&gt;sqlvar[0].sqllen  = 8;

  376. 

  377. int intval = 1;

  378. sqlda2-&gt;sqlvar[1].sqltype = ECPGt_int;

  379. sqlda2-&gt;sqlvar[1].sqldata = (char *) &amp;intval;

  380. sqlda2-&gt;sqlvar[1].sqllen  = sizeof(intval);

  381. </pre><p>

  382.     </p><p>

  383.      By opening a cursor and specifying the SQLDA that was set up

  384.      beforehand, the input parameters are passed to the prepared

  385.      statement.

  386. </p><pre class="programlisting">

  387. EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;

  388. </pre><p>

  389.     </p><p>

  390.      Finally, after using input SQLDAs, the allocated memory space

  391.      must be freed explicitly, unlike SQLDAs used for receiving query

  392.      results.

  393. </p><pre class="programlisting">

  394. free(sqlda2);

  395. </pre><p>

  396.     </p></div><div class="sect3" id="ECPG-SQLDA-EXAMPLE"><div class="titlepage"><div><div><h4 class="title">35.7.2.4. A Sample Application Using SQLDA</h4></div></div></div><p>

  397.      Here is an example program, which describes how to fetch access

  398.      statistics of the databases, specified by the input parameters,

  399.      from the system catalogs.

  400.     </p><p>

  401.      This application joins two system tables, pg_database and

  402.      pg_stat_database on the database OID, and also fetches and shows

  403.      the database statistics which are retrieved by two input

  404.      parameters (a database <code class="literal">postgres</code>, and OID <code class="literal">1</code>).

  405.     </p><p>

  406.      First, declare an SQLDA for input and an SQLDA for output.

  407. </p><pre class="programlisting">

  408. EXEC SQL include sqlda.h;

  409. 

  410. sqlda_t *sqlda1; /* an output descriptor */

  411. sqlda_t *sqlda2; /* an input descriptor  */

  412. </pre><p>

  413.     </p><p>

  414.      Next, connect to the database, prepare a statement, and declare a

  415.      cursor for the prepared statement.

  416. </p><pre class="programlisting">

  417. int

  418. main(void)

  419. {

  420.     EXEC SQL BEGIN DECLARE SECTION;

  421.     char query[1024] = "SELECT d.oid,* FROM pg_database d, pg_stat_database s WHERE d.oid=s.datid AND ( d.datname=? OR d.oid=? )";

  422.     EXEC SQL END DECLARE SECTION;

  423. 

  424.     EXEC SQL CONNECT TO testdb AS con1 USER testuser;

  425.     EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;

  426. 

  427.     EXEC SQL PREPARE stmt1 FROM :query;

  428.     EXEC SQL DECLARE cur1 CURSOR FOR stmt1;

  429. </pre><p>

  430.     </p><p>

  431.      Next, put some values in the input SQLDA for the input

  432.      parameters.  Allocate memory for the input SQLDA, and set the

  433.      number of input parameters to <code class="literal">sqln</code>.  Store

  434.      type, value, and value length into <code class="literal">sqltype</code>,

  435.      <code class="literal">sqldata</code>, and <code class="literal">sqllen</code> in the

  436.      <code class="literal">sqlvar</code> structure.

  437. 

  438. </p><pre class="programlisting">

  439.     /* Create SQLDA structure for input parameters. */

  440.     sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));

  441.     memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));

  442.     sqlda2-&gt;sqln = 2; /* number of input variables */

  443. 

  444.     sqlda2-&gt;sqlvar[0].sqltype = ECPGt_char;

  445.     sqlda2-&gt;sqlvar[0].sqldata = "postgres";

  446.     sqlda2-&gt;sqlvar[0].sqllen  = 8;

  447. 

  448.     intval = 1;

  449.     sqlda2-&gt;sqlvar[1].sqltype = ECPGt_int;

  450.     sqlda2-&gt;sqlvar[1].sqldata = (char *)&amp;intval;

  451.     sqlda2-&gt;sqlvar[1].sqllen  = sizeof(intval);

  452. </pre><p>

  453.     </p><p>

  454.      After setting up the input SQLDA, open a cursor with the input

  455.      SQLDA.

  456. 

  457. </p><pre class="programlisting">

  458.     /* Open a cursor with input parameters. */

  459.     EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;

  460. </pre><p>

  461.     </p><p>

  462.      Fetch rows into the output SQLDA from the opened cursor.

  463.      (Generally, you have to call <code class="command">FETCH</code> repeatedly

  464.      in the loop, to fetch all rows in the result set.)

  465. </p><pre class="programlisting">

  466.     while (1)

  467.     {

  468.         sqlda_t *cur_sqlda;

  469. 

  470.         /* Assign descriptor to the cursor  */

  471.         EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;

  472. </pre><p>

  473.     </p><p>

  474.      Next, retrieve the fetched records from the SQLDA, by following

  475.      the linked list of the <code class="type">sqlda_t</code> structure.

  476. </p><pre class="programlisting">

  477.     for (cur_sqlda = sqlda1 ;

  478.          cur_sqlda != NULL ;

  479.          cur_sqlda = cur_sqlda-&gt;desc_next)

  480.     {

  481.         ...

  482. </pre><p>

  483.     </p><p>

  484.      Read each columns in the first record.  The number of columns is

  485.      stored in <code class="structfield">sqld</code>, the actual data of the first

  486.      column is stored in <code class="literal">sqlvar[0]</code>, both members of

  487.      the <code class="type">sqlda_t</code> structure.

  488. 

  489. </p><pre class="programlisting">

  490.         /* Print every column in a row. */

  491.         for (i = 0; i &lt; sqlda1-&gt;sqld; i++)

  492.         {

  493.             sqlvar_t v = sqlda1-&gt;sqlvar[i];

  494.             char *sqldata = v.sqldata;

  495.             short sqllen  = v.sqllen;

  496. 

  497.             strncpy(name_buf, v.sqlname.data, v.sqlname.length);

  498.             name_buf[v.sqlname.length] = '\0';

  499. </pre><p>

  500.     </p><p>

  501.      Now, the column data is stored in the variable <code class="varname">v</code>.

  502.      Copy every datum into host variables, looking

  503.      at <code class="literal">v.sqltype</code> for the type of the column.

  504. </p><pre class="programlisting">

  505.             switch (v.sqltype) {

  506.                 int intval;

  507.                 double doubleval;

  508.                 unsigned long long int longlongval;

  509. 

  510.                 case ECPGt_char:

  511.                     memset(&amp;var_buf, 0, sizeof(var_buf));

  512.                     memcpy(&amp;var_buf, sqldata, (sizeof(var_buf) &lt;= sqllen ? sizeof(var_buf)-1 : sqllen));

  513.                     break;

  514. 

  515.                 case ECPGt_int: /* integer */

  516.                     memcpy(&amp;intval, sqldata, sqllen);

  517.                     snprintf(var_buf, sizeof(var_buf), "%d", intval);

  518.                     break;

  519. 

  520.                 ...

  521. 

  522.                 default:

  523.                     ...

  524.             }

  525. 

  526.             printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);

  527.         }

  528. </pre><p>

  529.     </p><p>

  530.      Close the cursor after processing all of records, and disconnect

  531.      from the database.

  532. </p><pre class="programlisting">

  533.     EXEC SQL CLOSE cur1;

  534.     EXEC SQL COMMIT;

  535. 

  536.     EXEC SQL DISCONNECT ALL;

  537. </pre><p>

  538.     </p><p>

  539.      The whole program is shown

  540.      in <a class="xref" href="ecpg-descriptors.html#ECPG-SQLDA-EXAMPLE-EXAMPLE" title="Example 35.1. Example SQLDA Program">Example 35.1</a>.

  541.     </p><div class="example" id="ECPG-SQLDA-EXAMPLE-EXAMPLE"><p class="title"><strong>Example 35.1. Example SQLDA Program</strong></p><div class="example-contents"><pre class="programlisting">

  542. #include &lt;stdlib.h&gt;

  543. #include &lt;string.h&gt;

  544. #include &lt;stdlib.h&gt;

  545. #include &lt;stdio.h&gt;

  546. #include &lt;unistd.h&gt;

  547. 

  548. EXEC SQL include sqlda.h;

  549. 

  550. sqlda_t *sqlda1; /* descriptor for output */

  551. sqlda_t *sqlda2; /* descriptor for input */

  552. 

  553. EXEC SQL WHENEVER NOT FOUND DO BREAK;

  554. EXEC SQL WHENEVER SQLERROR STOP;

  555. 

  556. int

  557. main(void)

  558. {

  559.     EXEC SQL BEGIN DECLARE SECTION;

  560.     char query[1024] = "SELECT d.oid,* FROM pg_database d, pg_stat_database s WHERE d.oid=s.datid AND ( d.datname=? OR d.oid=? )";

  561. 

  562.     int intval;

  563.     unsigned long long int longlongval;

  564.     EXEC SQL END DECLARE SECTION;

  565. 

  566.     EXEC SQL CONNECT TO uptimedb AS con1 USER uptime;

  567.     EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;

  568. 

  569.     EXEC SQL PREPARE stmt1 FROM :query;

  570.     EXEC SQL DECLARE cur1 CURSOR FOR stmt1;

  571. 

  572.     /* Create a SQLDA structure for an input parameter */

  573.     sqlda2 = (sqlda_t *)malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));

  574.     memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));

  575.     sqlda2-&gt;sqln = 2; /* a number of input variables */

  576. 

  577.     sqlda2-&gt;sqlvar[0].sqltype = ECPGt_char;

  578.     sqlda2-&gt;sqlvar[0].sqldata = "postgres";

  579.     sqlda2-&gt;sqlvar[0].sqllen  = 8;

  580. 

  581.     intval = 1;

  582.     sqlda2-&gt;sqlvar[1].sqltype = ECPGt_int;

  583.     sqlda2-&gt;sqlvar[1].sqldata = (char *) &amp;intval;

  584.     sqlda2-&gt;sqlvar[1].sqllen  = sizeof(intval);

  585. 

  586.     /* Open a cursor with input parameters. */

  587.     EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;

  588. 

  589.     while (1)

  590.     {

  591.         sqlda_t *cur_sqlda;

  592. 

  593.         /* Assign descriptor to the cursor  */

  594.         EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;

  595. 

  596.         for (cur_sqlda = sqlda1 ;

  597.              cur_sqlda != NULL ;

  598.              cur_sqlda = cur_sqlda-&gt;desc_next)

  599.         {

  600.             int i;

  601.             char name_buf[1024];

  602.             char var_buf[1024];

  603. 

  604.             /* Print every column in a row. */

  605.             for (i=0 ; i&lt;cur_sqlda-&gt;sqld ; i++)

  606.             {

  607.                 sqlvar_t v = cur_sqlda-&gt;sqlvar[i];

  608.                 char *sqldata = v.sqldata;

  609.                 short sqllen  = v.sqllen;

  610. 

  611.                 strncpy(name_buf, v.sqlname.data, v.sqlname.length);

  612.                 name_buf[v.sqlname.length] = '\0';

  613. 

  614.                 switch (v.sqltype)

  615.                 {

  616.                     case ECPGt_char:

  617.                         memset(&amp;var_buf, 0, sizeof(var_buf));

  618.                         memcpy(&amp;var_buf, sqldata, (sizeof(var_buf)&lt;=sqllen ? sizeof(var_buf)-1 : sqllen) );

  619.                         break;

  620. 

  621.                     case ECPGt_int: /* integer */

  622.                         memcpy(&amp;intval, sqldata, sqllen);

  623.                         snprintf(var_buf, sizeof(var_buf), "%d", intval);

  624.                         break;

  625. 

  626.                     case ECPGt_long_long: /* bigint */

  627.                         memcpy(&amp;longlongval, sqldata, sqllen);

  628.                         snprintf(var_buf, sizeof(var_buf), "%lld", longlongval);

  629.                         break;

  630. 

  631.                     default:

  632.                     {

  633.                         int i;

  634.                         memset(var_buf, 0, sizeof(var_buf));

  635.                         for (i = 0; i &lt; sqllen; i++)

  636.                         {

  637.                             char tmpbuf[16];

  638.                             snprintf(tmpbuf, sizeof(tmpbuf), "%02x ", (unsigned char) sqldata[i]);

  639.                             strncat(var_buf, tmpbuf, sizeof(var_buf));

  640.                         }

  641.                     }

  642.                         break;

  643.                 }

  644. 

  645.                 printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);

  646.             }

  647. 

  648.             printf("\n");

  649.         }

  650.     }

  651. 

  652.     EXEC SQL CLOSE cur1;

  653.     EXEC SQL COMMIT;

  654. 

  655.     EXEC SQL DISCONNECT ALL;

  656. 

  657.     return 0;

  658. }

  659. </pre><p>

  660.       The output of this example should look something like the

  661.       following (some numbers will vary).

  662.      </p><pre class="screen">

  663. oid = 1 (type: 1)

  664. datname = template1 (type: 1)

  665. datdba = 10 (type: 1)

  666. encoding = 0 (type: 5)

  667. datistemplate = t (type: 1)

  668. datallowconn = t (type: 1)

  669. datconnlimit = -1 (type: 5)

  670. datlastsysoid = 11510 (type: 1)

  671. datfrozenxid = 379 (type: 1)

  672. dattablespace = 1663 (type: 1)

  673. datconfig =  (type: 1)

  674. datacl = {=c/uptime,uptime=CTc/uptime} (type: 1)

  675. datid = 1 (type: 1)

  676. datname = template1 (type: 1)

  677. numbackends = 0 (type: 5)

  678. xact_commit = 113606 (type: 9)

  679. xact_rollback = 0 (type: 9)

  680. blks_read = 130 (type: 9)

  681. blks_hit = 7341714 (type: 9)

  682. tup_returned = 38262679 (type: 9)

  683. tup_fetched = 1836281 (type: 9)

  684. tup_inserted = 0 (type: 9)

  685. tup_updated = 0 (type: 9)

  686. tup_deleted = 0 (type: 9)

  687. 

  688. oid = 11511 (type: 1)

  689. datname = postgres (type: 1)

  690. datdba = 10 (type: 1)

  691. encoding = 0 (type: 5)

  692. datistemplate = f (type: 1)

  693. datallowconn = t (type: 1)

  694. datconnlimit = -1 (type: 5)

  695. datlastsysoid = 11510 (type: 1)

  696. datfrozenxid = 379 (type: 1)

  697. dattablespace = 1663 (type: 1)

  698. datconfig =  (type: 1)

  699. datacl =  (type: 1)

  700. datid = 11511 (type: 1)

  701. datname = postgres (type: 1)

  702. numbackends = 0 (type: 5)

  703. xact_commit = 221069 (type: 9)

  704. xact_rollback = 18 (type: 9)

  705. blks_read = 1176 (type: 9)

  706. blks_hit = 13943750 (type: 9)

  707. tup_returned = 77410091 (type: 9)

  708. tup_fetched = 3253694 (type: 9)

  709. tup_inserted = 0 (type: 9)

  710. tup_updated = 0 (type: 9)

  711. tup_deleted = 0 (type: 9)

  712. </pre></div></div><br class="example-break" /></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-pgtypes.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-errors.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">35.6. pgtypes Library </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 35.8. Error Handling</td></tr></table></div></body></html>
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