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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ECPG-DESCRIPTORS"
>33.7. Using Descriptor Areas</A
></H1
><P
> An SQL descriptor area is a more sophisticated method for processing
the result of a <TT
CLASS="COMMAND"
>SELECT</TT
>, <TT
CLASS="COMMAND"
>FETCH</TT
> or
a <TT
CLASS="COMMAND"
>DESCRIBE</TT
> statement. An SQL descriptor area groups
the data of one row of data together with metadata items into one
data structure. The metadata is particularly useful when executing
dynamic SQL statements, where the nature of the result columns might
not be known ahead of time. PostgreSQL provides two ways to use
Descriptor Areas: the named SQL Descriptor Areas and the C-structure
SQLDAs.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-NAMED-DESCRIPTORS"
>33.7.1. Named SQL Descriptor Areas</A
></H2
><P
> A named SQL descriptor area consists of a header, which contains
information concerning the entire descriptor, and one or more item
descriptor areas, which basically each describe one column in the
result row.
</P
><P
> Before you can use an SQL descriptor area, you need to allocate one:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL ALLOCATE DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>identifier</I
></TT
>;</PRE
><P>
The identifier serves as the <SPAN
CLASS="QUOTE"
>"variable name"</SPAN
> of the
descriptor area.
When you don't need the descriptor anymore, you should deallocate
it:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEALLOCATE DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>identifier</I
></TT
>;</PRE
><P>
</P
><P
> To use a descriptor area, specify it as the storage target in an
<TT
CLASS="LITERAL"
>INTO</TT
> clause, instead of listing host variables:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL FETCH NEXT FROM mycursor INTO SQL DESCRIPTOR mydesc;</PRE
><P>
If the result set is empty, the Descriptor Area will still contain
the metadata from the query, i.e. the field names.
</P
><P
> For not yet executed prepared queries, the <TT
CLASS="COMMAND"
>DESCRIBE</TT
>
statement can be used to get the metadata of the result set:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
char *sql_stmt = "SELECT * FROM table1";
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE stmt1 FROM :sql_stmt;
EXEC SQL DESCRIBE stmt1 INTO SQL DESCRIPTOR mydesc;</PRE
><P>
</P
><P
> Before PostgreSQL 9.0, the <TT
CLASS="LITERAL"
>SQL</TT
> keyword was optional,
so using <TT
CLASS="LITERAL"
>DESCRIPTOR</TT
> and <TT
CLASS="LITERAL"
>SQL DESCRIPTOR</TT
>
produced named SQL Descriptor Areas. Now it is mandatory, omitting
the <TT
CLASS="LITERAL"
>SQL</TT
> keyword produces SQLDA Descriptor Areas,
see <A
HREF="ecpg-descriptors.html#ECPG-SQLDA-DESCRIPTORS"
>Section 33.7.2</A
>.
</P
><P
> In <TT
CLASS="COMMAND"
>DESCRIBE</TT
> and <TT
CLASS="COMMAND"
>FETCH</TT
> statements,
the <TT
CLASS="LITERAL"
>INTO</TT
> and <TT
CLASS="LITERAL"
>USING</TT
> keywords can be
used to similarly: they produce the result set and the metadata in a
Descriptor Area.
</P
><P
> Now how do you get the data out of the descriptor area? You can
think of the descriptor area as a structure with named fields. To
retrieve the value of a field from the header and store it into a
host variable, use the following command:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL GET DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> :<TT
CLASS="REPLACEABLE"
><I
>hostvar</I
></TT
> = <TT
CLASS="REPLACEABLE"
><I
>field</I
></TT
>;</PRE
><P>
Currently, there is only one header field defined:
<TT
CLASS="REPLACEABLE"
><I
>COUNT</I
></TT
>, which tells how many item
descriptor areas exist (that is, how many columns are contained in
the result). The host variable needs to be of an integer type. To
get a field from the item descriptor area, use the following
command:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL GET DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> VALUE <TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> :<TT
CLASS="REPLACEABLE"
><I
>hostvar</I
></TT
> = <TT
CLASS="REPLACEABLE"
><I
>field</I
></TT
>;</PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> can be a literal integer or a host
variable containing an integer. Possible fields are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>CARDINALITY</TT
> (integer)</DT
><DD
><P
> number of rows in the result set
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATA</TT
></DT
><DD
><P
> actual data item (therefore, the data type of this field
depends on the query)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATETIME_INTERVAL_CODE</TT
> (integer)</DT
><DD
><P
> When <TT
CLASS="LITERAL"
>TYPE</TT
> is <TT
CLASS="LITERAL"
>9</TT
>,
<TT
CLASS="LITERAL"
>DATETIME_INTERVAL_CODE</TT
> will have a value of
<TT
CLASS="LITERAL"
>1</TT
> for <TT
CLASS="LITERAL"
>DATE</TT
>,
<TT
CLASS="LITERAL"
>2</TT
> for <TT
CLASS="LITERAL"
>TIME</TT
>,
<TT
CLASS="LITERAL"
>3</TT
> for <TT
CLASS="LITERAL"
>TIMESTAMP</TT
>,
<TT
CLASS="LITERAL"
>4</TT
> for <TT
CLASS="LITERAL"
>TIME WITH TIME ZONE</TT
>, or
<TT
CLASS="LITERAL"
>5</TT
> for <TT
CLASS="LITERAL"
>TIMESTAMP WITH TIME ZONE</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATETIME_INTERVAL_PRECISION</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>INDICATOR</TT
> (integer)</DT
><DD
><P
> the indicator (indicating a null value or a value truncation)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>KEY_MEMBER</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>LENGTH</TT
> (integer)</DT
><DD
><P
> length of the datum in characters
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NAME</TT
> (string)</DT
><DD
><P
> name of the column
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NULLABLE</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>OCTET_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the character representation of the datum in bytes
</P
></DD
><DT
><TT
CLASS="LITERAL"
>PRECISION</TT
> (integer)</DT
><DD
><P
> precision (for type <TT
CLASS="TYPE"
>numeric</TT
>)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>RETURNED_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the datum in characters
</P
></DD
><DT
><TT
CLASS="LITERAL"
>RETURNED_OCTET_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the character representation of the datum in bytes
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SCALE</TT
> (integer)</DT
><DD
><P
> scale (for type <TT
CLASS="TYPE"
>numeric</TT
>)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TYPE</TT
> (integer)</DT
><DD
><P
> numeric code of the data type of the column
</P
></DD
></DL
></DIV
><P>
</P
><P
> In <TT
CLASS="COMMAND"
>EXECUTE</TT
>, <TT
CLASS="COMMAND"
>DECLARE</TT
> and <TT
CLASS="COMMAND"
>OPEN</TT
>
statements, the effect of the <TT
CLASS="LITERAL"
>INTO</TT
> and <TT
CLASS="LITERAL"
>USING</TT
>
keywords are different. A Descriptor Area can also be manually built to
provide the input parameters for a query or a cursor and
<TT
CLASS="LITERAL"
>USING SQL DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
></TT
>
is the way to pass the input parameters into a parametrized query. The statement
to build a named SQL Descriptor Area is below:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL SET DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> VALUE <TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>field</I
></TT
> = :<TT
CLASS="REPLACEABLE"
><I
>hostvar</I
></TT
>;</PRE
><P>
</P
><P
> PostgreSQL supports retrieving more that one record in one <TT
CLASS="COMMAND"
>FETCH</TT
>
statement and storing the data in host variables in this case assumes that the
variable is an array. E.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
int id[5];
EXEC SQL END DECLARE SECTION;
EXEC SQL FETCH 5 FROM mycursor INTO SQL DESCRIPTOR mydesc;
EXEC SQL GET DESCRIPTOR mydesc VALUE 1 :id = DATA;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="ECPG-SQLDA-DESCRIPTORS"
>33.7.2. SQLDA Descriptor Areas</A
></H2
><P
> An SQLDA Descriptor Area is a C language structure which can be also used
to get the result set and the metadata of a query. One structure stores one
record from the result set.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL include sqlda.h;
sqlda_t *mysqlda;
EXEC SQL FETCH 3 FROM mycursor INTO DESCRIPTOR mysqlda;</PRE
><P>
Note that the <TT
CLASS="LITERAL"
>SQL</TT
> keyword is omitted. The paragraphs about
the use cases of the <TT
CLASS="LITERAL"
>INTO</TT
> and <TT
CLASS="LITERAL"
>USING</TT
>
keywords in <A
HREF="ecpg-descriptors.html#ECPG-NAMED-DESCRIPTORS"
>Section 33.7.1</A
> also apply here with an addition.
In a <TT
CLASS="COMMAND"
>DESCRIBE</TT
> statement the <TT
CLASS="LITERAL"
>DESCRIPTOR</TT
>
keyword can be completely omitted if the <TT
CLASS="LITERAL"
>INTO</TT
> keyword is used:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DESCRIBE prepared_statement INTO mysqlda;</PRE
><P>
</P
><DIV
CLASS="PROCEDURE"
><P
> The general flow of a program that uses SQLDA is:
</P
><OL
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"
><H3
CLASS="SECT3"
><A
NAME="AEN44211"
>33.7.2.1. SQLDA Data Structure</A
></H3
><P
> SQLDA uses three data structure
types: <TT
CLASS="TYPE"
>sqlda_t</TT
>, <TT
CLASS="TYPE"
>sqlvar_t</TT
>,
and <TT
CLASS="TYPE"
>struct sqlname</TT
>.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> PostgreSQL's SQLDA has a similar data structure to the one in
IBM DB2 Universal Database, so some technical information on
DB2's SQLDA could help understanding PostgreSQL's one better.
</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-SQLDA-SQLDA"
>33.7.2.1.1. sqlda_t Structure</A
></H4
><P
> The structure type <TT
CLASS="TYPE"
>sqlda_t</TT
> is the type of the
actual SQLDA. It holds one record. And two or
more <TT
CLASS="TYPE"
>sqlda_t</TT
> structures can be connected in a
linked list with the pointer in
the <TT
CLASS="STRUCTFIELD"
>desc_next</TT
> field, thus
representing an ordered collection of rows. So, when two or
more rows are fetched, the application can read them by
following the <TT
CLASS="STRUCTFIELD"
>desc_next</TT
> pointer in
each <TT
CLASS="TYPE"
>sqlda_t</TT
> node.
</P
><P
> The definition of <TT
CLASS="TYPE"
>sqlda_t</TT
> is:
</P><PRE
CLASS="PROGRAMLISTING"
>struct sqlda_struct
{
char sqldaid[8];
long sqldabc;
short sqln;
short sqld;
struct sqlda_struct *desc_next;
struct sqlvar_struct sqlvar[1];
};
typedef struct sqlda_struct sqlda_t;</PRE
><P>
The meaning of the fields is:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>sqldaid</TT
></DT
><DD
><P
> It contains the literal string <TT
CLASS="LITERAL"
>"SQLDA "</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqldabc</TT
></DT
><DD
><P
> It contains the size of the allocated space in bytes.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqln</TT
></DT
><DD
><P
> It contains the number of input parameters for a parametrized query in
case it's passed into <TT
CLASS="COMMAND"
>OPEN</TT
>, <TT
CLASS="COMMAND"
>DECLARE</TT
> or
<TT
CLASS="COMMAND"
>EXECUTE</TT
> statements using the <TT
CLASS="LITERAL"
>USING</TT
>
keyword. In case it's used as output of <TT
CLASS="COMMAND"
>SELECT</TT
>,
<TT
CLASS="COMMAND"
>EXECUTE</TT
> or <TT
CLASS="COMMAND"
>FETCH</TT
> statements,
its value is the same as <TT
CLASS="LITERAL"
>sqld</TT
>
statement
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqld</TT
></DT
><DD
><P
> It contains the number of fields in a result set.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>desc_next</TT
></DT
><DD
><P
> If the query returns more than one record, multiple linked
SQLDA structures are returned, and <TT
CLASS="LITERAL"
>desc_next</TT
> holds
a pointer to the next entry in the list.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqlvar</TT
></DT
><DD
><P
> This is the array of the columns in the result set.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-SQLDA-SQLVAR"
>33.7.2.1.2. sqlvar_t Structure</A
></H4
><P
> The structure type <TT
CLASS="TYPE"
>sqlvar_t</TT
> holds a column value
and metadata such as type and length. The definition of the type
is:
</P><PRE
CLASS="PROGRAMLISTING"
>struct sqlvar_struct
{
short sqltype;
short sqllen;
char *sqldata;
short *sqlind;
struct sqlname sqlname;
};
typedef struct sqlvar_struct sqlvar_t;</PRE
><P>
The meaning of the fields is:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>sqltype</TT
></DT
><DD
><P
> Contains the type identifier of the field. For values,
see <TT
CLASS="LITERAL"
>enum ECPGttype</TT
> in <TT
CLASS="LITERAL"
>ecpgtype.h</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqllen</TT
></DT
><DD
><P
> Contains the binary length of the field. e.g. 4 bytes for <TT
CLASS="TYPE"
>ECPGt_int</TT
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqldata</TT
></DT
><DD
><P
> Points to the data. The format of the data is described
in <A
HREF="ecpg-variables.html#ECPG-VARIABLES-TYPE-MAPPING"
>Section 33.4.4</A
>.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqlind</TT
></DT
><DD
><P
> Points to the null indicator. 0 means not null, -1 means
null.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>sqlname</TT
></DT
><DD
><P
> The name of the field.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
><DIV
CLASS="SECT4"
><H4
CLASS="SECT4"
><A
NAME="ECPG-SQLDA-SQLNAME"
>33.7.2.1.3. struct sqlname Structure</A
></H4
><P
> A <TT
CLASS="TYPE"
>struct sqlname</TT
> structure holds a column name. It
is used as a member of the <TT
CLASS="TYPE"
>sqlvar_t</TT
> structure. The
definition of the structure is:
</P><PRE
CLASS="PROGRAMLISTING"
>#define NAMEDATALEN 64
struct sqlname
{
short length;
char data[NAMEDATALEN];
};</PRE
><P>
The meaning of the fields is:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>length</TT
></DT
><DD
><P
> Contains the length of the field name.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>data</TT
></DT
><DD
><P
> Contains the actual field name.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-SQLDA-OUTPUT"
>33.7.2.2. Retrieving a Result Set Using an SQLDA</A
></H3
><DIV
CLASS="PROCEDURE"
><P
> The general steps to retrieve a query result set through an
SQLDA are:
</P
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
>Declare an <TT
CLASS="TYPE"
>sqlda_t</TT
> structure to receive the result set.</P
></LI
><LI
CLASS="STEP"
><P
>Execute <TT
CLASS="COMMAND"
>FETCH</TT
>/<TT
CLASS="COMMAND"
>EXECUTE</TT
>/<TT
CLASS="COMMAND"
>DESCRIBE</TT
> 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 <TT
CLASS="STRUCTFIELD"
>sqln</TT
>, a member of the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure.</P
></LI
><LI
CLASS="STEP"
><P
>Get the values of each column from <TT
CLASS="LITERAL"
>sqlvar[0]</TT
>, <TT
CLASS="LITERAL"
>sqlvar[1]</TT
>, etc., members of the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure.</P
></LI
><LI
CLASS="STEP"
><P
>Go to next row (<TT
CLASS="TYPE"
>sqlda_t</TT
> structure) by following the <TT
CLASS="STRUCTFIELD"
>desc_next</TT
> pointer, a member of the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure.</P
></LI
><LI
CLASS="STEP"
><P
>Repeat above as you need.</P
></LI
></OL
></DIV
><P
> Here is an example retrieving a result set through an SQLDA.
</P
><P
> First, declare a <TT
CLASS="TYPE"
>sqlda_t</TT
> structure to receive the result set.
</P><PRE
CLASS="PROGRAMLISTING"
>sqlda_t *sqlda1;</PRE
><P>
</P
><P
> Next, specify the SQLDA in a command. This is
a <TT
CLASS="COMMAND"
>FETCH</TT
> command example.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;</PRE
><P>
</P
><P
> Run a loop following the linked list to retrieve the rows.
</P><PRE
CLASS="PROGRAMLISTING"
>sqlda_t *cur_sqlda;
for (cur_sqlda = sqlda1;
cur_sqlda != NULL;
cur_sqlda = cur_sqlda->desc_next)
{
...
}</PRE
><P>
</P
><P
> Inside the loop, run another loop to retrieve each column data
(<TT
CLASS="TYPE"
>sqlvar_t</TT
> structure) of the row.
</P><PRE
CLASS="PROGRAMLISTING"
>for (i = 0; i < cur_sqlda->sqld; i++)
{
sqlvar_t v = cur_sqlda->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
...
}</PRE
><P>
</P
><P
> To get a column value, check the <TT
CLASS="STRUCTFIELD"
>sqltype</TT
> value,
a member of the <TT
CLASS="TYPE"
>sqlvar_t</TT
> structure. Then, switch
to an appropriate way, depending on the column type, to copy
data from the <TT
CLASS="STRUCTFIELD"
>sqlvar</TT
> field to a host variable.
</P><PRE
CLASS="PROGRAMLISTING"
>char var_buf[1024];
switch (v.sqltype)
{
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf) <= sqllen ? sizeof(var_buf) - 1 : sqllen));
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
...
}</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-SQLDA-INPUT"
>33.7.2.3. Passing Query Parameters Using an SQLDA</A
></H3
><DIV
CLASS="PROCEDURE"
><P
> The general steps to use an SQLDA to pass input
parameters to a prepared query are:
</P
><OL
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
> Here is an example.
</P
><P
> First, create a prepared statement.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL BEGIN DECLARE SECTION;
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 = ?)";
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE stmt1 FROM :query;</PRE
><P>
</P
><P
> Next, allocate memory for an SQLDA, and set the number of input
parameters in <TT
CLASS="STRUCTFIELD"
>sqln</TT
>, a member variable of
the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure. When two or more input
parameters are required for the prepared query, the application
has to allocate additional memory space which is calculated by
(nr. of params - 1) * sizeof(sqlvar_t). The example shown here
allocates memory space for two input parameters.
</P><PRE
CLASS="PROGRAMLISTING"
>sqlda_t *sqlda2;
sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* number of input variables */</PRE
><P>
</P
><P
> After memory allocation, store the parameter values into the
<TT
CLASS="LITERAL"
>sqlvar[]</TT
> array. (This is same array used for
retrieving column values when the SQLDA is receiving a result
set.) In this example, the input parameters
are <TT
CLASS="LITERAL"
>"postgres"</TT
>, having a string type,
and <TT
CLASS="LITERAL"
>1</TT
>, having an integer type.
</P><PRE
CLASS="PROGRAMLISTING"
>sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
int intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *) &intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);</PRE
><P>
</P
><P
> By opening a cursor and specifying the SQLDA that was set up
beforehand, the input parameters are passed to the prepared
statement.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;</PRE
><P>
</P
><P
> Finally, after using input SQLDAs, the allocated memory space
must be freed explicitly, unlike SQLDAs used for receiving query
results.
</P><PRE
CLASS="PROGRAMLISTING"
>free(sqlda2);</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="ECPG-SQLDA-EXAMPLE"
>33.7.2.4. A Sample Application Using SQLDA</A
></H3
><P
> Here is an example program, which describes how to fetch access
statistics of the databases, specified by the input parameters,
from the system catalogs.
</P
><P
> This application joins two system tables, pg_database and
pg_stat_database on the database OID, and also fetches and shows
the database statistics which are retrieved by two input
parameters (a database <TT
CLASS="LITERAL"
>postgres</TT
>, and OID <TT
CLASS="LITERAL"
>1</TT
>).
</P
><P
> First, declare an SQLDA for input and an SQLDA for output.
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL include sqlda.h;
sqlda_t *sqlda1; /* an output descriptor */
sqlda_t *sqlda2; /* an input descriptor */</PRE
><P>
</P
><P
> Next, connect to the database, prepare a statement, and declare a
cursor for the prepared statement.
</P><PRE
CLASS="PROGRAMLISTING"
>int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
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=? )";
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;</PRE
><P>
</P
><P
> Next, put some values in the input SQLDA for the input
parameters. Allocate memory for the input SQLDA, and set the
number of input parameters to <TT
CLASS="LITERAL"
>sqln</TT
>. Store
type, value, and value length into <TT
CLASS="LITERAL"
>sqltype</TT
>,
<TT
CLASS="LITERAL"
>sqldata</TT
>, and <TT
CLASS="LITERAL"
>sqllen</TT
> in the
<TT
CLASS="LITERAL"
>sqlvar</TT
> structure.
</P><PRE
CLASS="PROGRAMLISTING"
> /* Create SQLDA structure for input parameters. */
sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* number of input variables */
sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *)&intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);</PRE
><P>
</P
><P
> After setting up the input SQLDA, open a cursor with the input
SQLDA.
</P><PRE
CLASS="PROGRAMLISTING"
> /* Open a cursor with input parameters. */
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;</PRE
><P>
</P
><P
> Fetch rows into the output SQLDA from the opened cursor.
(Generally, you have to call <TT
CLASS="COMMAND"
>FETCH</TT
> repeatedly
in the loop, to fetch all rows in the result set.)
</P><PRE
CLASS="PROGRAMLISTING"
> while (1)
{
sqlda_t *cur_sqlda;
/* Assign descriptor to the cursor */
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;</PRE
><P>
</P
><P
> Next, retrieve the fetched records from the SQLDA, by following
the linked list of the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure.
</P><PRE
CLASS="PROGRAMLISTING"
> for (cur_sqlda = sqlda1 ;
cur_sqlda != NULL ;
cur_sqlda = cur_sqlda->desc_next)
{
...</PRE
><P>
</P
><P
> Read each columns in the first record. The number of columns is
stored in <TT
CLASS="STRUCTFIELD"
>sqld</TT
>, the actual data of the first
column is stored in <TT
CLASS="LITERAL"
>sqlvar[0]</TT
>, both members of
the <TT
CLASS="TYPE"
>sqlda_t</TT
> structure.
</P><PRE
CLASS="PROGRAMLISTING"
> /* Print every column in a row. */
for (i = 0; i < sqlda1->sqld; i++)
{
sqlvar_t v = sqlda1->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
strncpy(name_buf, v.sqlname.data, v.sqlname.length);
name_buf[v.sqlname.length] = '\0';</PRE
><P>
</P
><P
> Now, the column data is stored in the variable <TT
CLASS="VARNAME"
>v</TT
>.
Copy every datum into host variables, looking
at <TT
CLASS="LITERAL"
>v.sqltype</TT
> for the type of the column.
</P><PRE
CLASS="PROGRAMLISTING"
> switch (v.sqltype) {
int intval;
double doubleval;
unsigned long long int longlongval;
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf) <= sqllen ? sizeof(var_buf)-1 : sqllen));
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
...
default:
...
}
printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);
}</PRE
><P>
</P
><P
> Close the cursor after processing all of records, and disconnect
from the database.
</P><PRE
CLASS="PROGRAMLISTING"
> EXEC SQL CLOSE cur1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;</PRE
><P>
</P
><P
> The whole program is shown
in <A
HREF="ecpg-descriptors.html#ECPG-SQLDA-EXAMPLE-EXAMPLE"
>Example 33-1</A
>.
</P
><DIV
CLASS="EXAMPLE"
><A
NAME="ECPG-SQLDA-EXAMPLE-EXAMPLE"
></A
><P
><B
>Example 33-1. Example SQLDA Program</B
></P
><PRE
CLASS="PROGRAMLISTING"
>#include <stdlib.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
EXEC SQL include sqlda.h;
sqlda_t *sqlda1; /* descriptor for output */
sqlda_t *sqlda2; /* descriptor for input */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
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=? )";
int intval;
unsigned long long int longlongval;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO uptimedb AS con1 USER uptime;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
/* Create a SQLDA structure for an input parameter */
sqlda2 = (sqlda_t *)malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* a number of input variables */
sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *) &intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);
/* Open a cursor with input parameters. */
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
while (1)
{
sqlda_t *cur_sqlda;
/* Assign descriptor to the cursor */
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
for (cur_sqlda = sqlda1 ;
cur_sqlda != NULL ;
cur_sqlda = cur_sqlda->desc_next)
{
int i;
char name_buf[1024];
char var_buf[1024];
/* Print every column in a row. */
for (i=0 ; i<cur_sqlda->sqld ; i++)
{
sqlvar_t v = cur_sqlda->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
strncpy(name_buf, v.sqlname.data, v.sqlname.length);
name_buf[v.sqlname.length] = '\0';
switch (v.sqltype)
{
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf)<=sqllen ? sizeof(var_buf)-1 : sqllen) );
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
case ECPGt_long_long: /* bigint */
memcpy(&longlongval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%lld", longlongval);
break;
default:
{
int i;
memset(var_buf, 0, sizeof(var_buf));
for (i = 0; i < sqllen; i++)
{
char tmpbuf[16];
snprintf(tmpbuf, sizeof(tmpbuf), "%02x ", (unsigned char) sqldata[i]);
strncat(var_buf, tmpbuf, sizeof(var_buf));
}
}
break;
}
printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);
}
printf("\n");
}
}
EXEC SQL CLOSE cur1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}</PRE
><P
> The output of this example should look something like the
following (some numbers will vary).
</P
><PRE
CLASS="SCREEN"
>oid = 1 (type: 1)
datname = template1 (type: 1)
datdba = 10 (type: 1)
encoding = 0 (type: 5)
datistemplate = t (type: 1)
datallowconn = t (type: 1)
datconnlimit = -1 (type: 5)
datlastsysoid = 11510 (type: 1)
datfrozenxid = 379 (type: 1)
dattablespace = 1663 (type: 1)
datconfig = (type: 1)
datacl = {=c/uptime,uptime=CTc/uptime} (type: 1)
datid = 1 (type: 1)
datname = template1 (type: 1)
numbackends = 0 (type: 5)
xact_commit = 113606 (type: 9)
xact_rollback = 0 (type: 9)
blks_read = 130 (type: 9)
blks_hit = 7341714 (type: 9)
tup_returned = 38262679 (type: 9)
tup_fetched = 1836281 (type: 9)
tup_inserted = 0 (type: 9)
tup_updated = 0 (type: 9)
tup_deleted = 0 (type: 9)
oid = 11511 (type: 1)
datname = postgres (type: 1)
datdba = 10 (type: 1)
encoding = 0 (type: 5)
datistemplate = f (type: 1)
datallowconn = t (type: 1)
datconnlimit = -1 (type: 5)
datlastsysoid = 11510 (type: 1)
datfrozenxid = 379 (type: 1)
dattablespace = 1663 (type: 1)
datconfig = (type: 1)
datacl = (type: 1)
datid = 11511 (type: 1)
datname = postgres (type: 1)
numbackends = 0 (type: 5)
xact_commit = 221069 (type: 9)
xact_rollback = 18 (type: 9)
blks_read = 1176 (type: 9)
blks_hit = 13943750 (type: 9)
tup_returned = 77410091 (type: 9)
tup_fetched = 3253694 (type: 9)
tup_inserted = 0 (type: 9)
tup_updated = 0 (type: 9)
tup_deleted = 0 (type: 9)</PRE
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