An SQL descriptor area is a more sophisticated method for processing
the result of a SELECT, FETCH or
a DESCRIBE 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.
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.
Before you can use an SQL descriptor area, you need to allocate one:
EXEC SQL ALLOCATE DESCRIPTOR identifier;
The identifier serves as the “variable name” of the descriptor area. When you don't need the descriptor anymore, you should deallocate it:
EXEC SQL DEALLOCATE DESCRIPTOR identifier;
To use a descriptor area, specify it as the storage target in an
INTO clause, instead of listing host variables:
EXEC SQL FETCH NEXT FROM mycursor INTO SQL DESCRIPTOR mydesc;
If the result set is empty, the Descriptor Area will still contain the metadata from the query, i.e., the field names.
For not yet executed prepared queries, the DESCRIBE
statement can be used to get the metadata of the result set:
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;
Before PostgreSQL 9.0, the SQL keyword was optional,
so using DESCRIPTOR and SQL DESCRIPTOR
produced named SQL Descriptor Areas. Now it is mandatory, omitting
the SQL keyword produces SQLDA Descriptor Areas,
see Section 38.7.2.
In DESCRIBE and FETCH statements,
the INTO and USING keywords can be
used to similarly: they produce the result set and the metadata in a
Descriptor Area.
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:
EXEC SQL GET DESCRIPTORname:hostvar=field;
Currently, there is only one header field defined:
COUNT, 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:
EXEC SQL GET DESCRIPTORnameVALUEnum:hostvar=field;
num can be a literal integer or a host
variable containing an integer. Possible fields are:
CARDINALITY (integer) #number of rows in the result set
DATA #actual data item (therefore, the data type of this field depends on the query)
DATETIME_INTERVAL_CODE (integer) #
When TYPE is 9,
DATETIME_INTERVAL_CODE will have a value of
1 for DATE,
2 for TIME,
3 for TIMESTAMP,
4 for TIME WITH TIME ZONE, or
5 for TIMESTAMP WITH TIME ZONE.
DATETIME_INTERVAL_PRECISION (integer) #not implemented
INDICATOR (integer) #the indicator (indicating a null value or a value truncation)
KEY_MEMBER (integer) #not implemented
LENGTH (integer) #length of the datum in characters
NAME (string) #name of the column
NULLABLE (integer) #not implemented
OCTET_LENGTH (integer) #length of the character representation of the datum in bytes
PRECISION (integer) #
precision (for type numeric)
RETURNED_LENGTH (integer) #length of the datum in characters
RETURNED_OCTET_LENGTH (integer) #length of the character representation of the datum in bytes
SCALE (integer) #
scale (for type numeric)
TYPE (integer) #numeric code of the data type of the column
In EXECUTE, DECLARE and OPEN
statements, the effect of the INTO and USING
keywords are different. A Descriptor Area can also be manually built to
provide the input parameters for a query or a cursor and
USING SQL DESCRIPTOR
is the way to pass the input parameters into a parameterized query. The statement
to build a named SQL Descriptor Area is below:
name
EXEC SQL SET DESCRIPTORnameVALUEnumfield= :hostvar;
PostgreSQL supports retrieving more that one record in one FETCH
statement and storing the data in host variables in this case assumes that the
variable is an array. E.g.:
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;
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.
EXEC SQL include sqlda.h; sqlda_t *mysqlda; EXEC SQL FETCH 3 FROM mycursor INTO DESCRIPTOR mysqlda;
Note that the SQL keyword is omitted. The paragraphs about
the use cases of the INTO and USING
keywords in Section 38.7.1 also apply here with an addition.
In a DESCRIBE statement the DESCRIPTOR
keyword can be completely omitted if the INTO keyword is used:
EXEC SQL DESCRIBE prepared_statement INTO mysqlda;
The general flow of a program that uses SQLDA is:
Prepare a query, and declare a cursor for it.
Declare an SQLDA for the result rows.
Declare an SQLDA for the input parameters, and initialize them (memory allocation, parameter settings).
Open a cursor with the input SQLDA.
Fetch rows from the cursor, and store them into an output SQLDA.
Read values from the output SQLDA into the host variables (with conversion if necessary).
Close the cursor.
Free the memory area allocated for the input SQLDA.
SQLDA uses three data structure
types: sqlda_t, sqlvar_t,
and struct sqlname.
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.
The structure type sqlda_t is the type of the
actual SQLDA. It holds one record. And two or
more sqlda_t structures can be connected in a
linked list with the pointer in
the desc_next field, thus
representing an ordered collection of rows. So, when two or
more rows are fetched, the application can read them by
following the desc_next pointer in
each sqlda_t node.
The definition of sqlda_t is:
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;
The meaning of the fields is:
sqldaid #
It contains the literal string "SQLDA ".
sqldabc #It contains the size of the allocated space in bytes.
sqln #
It contains the number of input parameters for a parameterized query in
case it's passed into OPEN, DECLARE or
EXECUTE statements using the USING
keyword. In case it's used as output of SELECT,
EXECUTE or FETCH statements,
its value is the same as sqld
statement
sqld #It contains the number of fields in a result set.
desc_next #
If the query returns more than one record, multiple linked
SQLDA structures are returned, and desc_next holds
a pointer to the next entry in the list.
sqlvar #This is the array of the columns in the result set.
The structure type sqlvar_t holds a column value
and metadata such as type and length. The definition of the type
is:
struct sqlvar_struct
{
short sqltype;
short sqllen;
char *sqldata;
short *sqlind;
struct sqlname sqlname;
};
typedef struct sqlvar_struct sqlvar_t;
The meaning of the fields is:
sqltype #
Contains the type identifier of the field. For values,
see enum ECPGttype in ecpgtype.h.
sqllen #
Contains the binary length of the field. e.g., 4 bytes for ECPGt_int.
sqldata #Points to the data. The format of the data is described in Section 38.4.4.
sqlind #Points to the null indicator. 0 means not null, -1 means null.
sqlname #The name of the field.
A struct sqlname structure holds a column name. It
is used as a member of the sqlvar_t structure. The
definition of the structure is:
#define NAMEDATALEN 64
struct sqlname
{
short length;
char data[NAMEDATALEN];
};
The meaning of the fields is:
The general steps to retrieve a query result set through an SQLDA are:
Declare an sqlda_t structure to receive the result set.
Execute FETCH/EXECUTE/DESCRIBE commands to process a query specifying the declared SQLDA.
Check the number of records in the result set by looking at sqln, a member of the sqlda_t structure.
Get the values of each column from sqlvar[0], sqlvar[1], etc., members of the sqlda_t structure.
Go to next row (sqlda_t structure) by following the desc_next pointer, a member of the sqlda_t structure.
Repeat above as you need.
Here is an example retrieving a result set through an SQLDA.
First, declare a sqlda_t structure to receive the result set.
sqlda_t *sqlda1;
Next, specify the SQLDA in a command. This is
a FETCH command example.
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
Run a loop following the linked list to retrieve the rows.
sqlda_t *cur_sqlda;
for (cur_sqlda = sqlda1;
cur_sqlda != NULL;
cur_sqlda = cur_sqlda->desc_next)
{
...
}
Inside the loop, run another loop to retrieve each column data
(sqlvar_t structure) of the row.
for (i = 0; i < cur_sqlda->sqld; i++)
{
sqlvar_t v = cur_sqlda->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
...
}
To get a column value, check the sqltype value,
a member of the sqlvar_t structure. Then, switch
to an appropriate way, depending on the column type, to copy
data from the sqlvar field to a host variable.
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;
...
}
The general steps to use an SQLDA to pass input parameters to a prepared query are:
Create a prepared query (prepared statement)
Declare an sqlda_t structure as an input SQLDA.
Allocate memory area (as sqlda_t structure) for the input SQLDA.
Set (copy) input values in the allocated memory.
Open a cursor with specifying the input SQLDA.
Here is an example.
First, create a prepared statement.
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;
Next, allocate memory for an SQLDA, and set the number of input
parameters in sqln, a member variable of
the sqlda_t 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.
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 */
After memory allocation, store the parameter values into the
sqlvar[] 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 "postgres", having a string type,
and 1, having an integer type.
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);
By opening a cursor and specifying the SQLDA that was set up beforehand, the input parameters are passed to the prepared statement.
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
Finally, after using input SQLDAs, the allocated memory space must be freed explicitly, unlike SQLDAs used for receiving query results.
free(sqlda2);
Here is an example program, which describes how to fetch access statistics of the databases, specified by the input parameters, from the system catalogs.
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 postgres, and OID 1).
First, declare an SQLDA for input and an SQLDA for output.
EXEC SQL include sqlda.h; sqlda_t *sqlda1; /* an output descriptor */ sqlda_t *sqlda2; /* an input descriptor */
Next, connect to the database, prepare a statement, and declare a cursor for the prepared statement.
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 SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
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 sqln. Store
type, value, and value length into sqltype,
sqldata, and sqllen in the
sqlvar structure.
/* 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);
After setting up the input SQLDA, open a cursor with the input SQLDA.
/* Open a cursor with input parameters. */
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
Fetch rows into the output SQLDA from the opened cursor.
(Generally, you have to call FETCH repeatedly
in the loop, to fetch all rows in the result set.)
while (1)
{
sqlda_t *cur_sqlda;
/* Assign descriptor to the cursor */
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
Next, retrieve the fetched records from the SQLDA, by following
the linked list of the sqlda_t structure.
for (cur_sqlda = sqlda1 ;
cur_sqlda != NULL ;
cur_sqlda = cur_sqlda->desc_next)
{
...
Read each columns in the first record. The number of columns is
stored in sqld, the actual data of the first
column is stored in sqlvar[0], both members of
the sqlda_t structure.
/* 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';
Now, the column data is stored in the variable v.
Copy every datum into host variables, looking
at v.sqltype for the type of the column.
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);
}
Close the cursor after processing all of records, and disconnect from the database.
EXEC SQL CLOSE cur1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
The whole program is shown in Example 38.1.
Example 38.1. Example SQLDA Program
#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 SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
/* Create an 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;
}
The output of this example should look something like the following (some numbers will vary).
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)
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)
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)