35.7. Using Descriptor Areas
Prev | Up | Chapter 35. ECPG - Embedded SQL in C | Home | Next

## 35.7. Using Descriptor Areas

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.

### 35.7.1. Named SQL Descriptor Areas

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 35.7.2.

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 DESCRIPTOR name :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 DESCRIPTOR name VALUE num :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 `name`` is the way to pass the input parameters into a parameterized query. The statement to build a named SQL Descriptor Area is below:

    EXEC SQL SET DESCRIPTOR name VALUE num field = :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;

### 35.7.2. SQLDA Descriptor Areas

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 35.7.1 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:

1. Prepare a query, and declare a cursor for it.

2. Declare an SQLDA for the result rows.

3. Declare an SQLDA for the input parameters, and initialize them (memory allocation, parameter settings).

4. Open a cursor with the input SQLDA.

5. Fetch rows from the cursor, and store them into an output SQLDA.

6. Read values from the output SQLDA into the host variables (with conversion if necessary).

7. Close the cursor.

8. Free the memory area allocated for the input SQLDA.

#### 35.7.2.1. SQLDA Data Structure

SQLDA uses three data structure types: `sqlda_t`, `sqlvar_t`, and `struct sqlname`.

### Tip

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.

##### 35.7.2.1.1. sqlda_t Structure

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.

##### 35.7.2.1.2. sqlvar_t Structure

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 35.4.4.

`sqlind`

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

`sqlname`

  The name of the field.

##### 35.7.2.1.3. struct sqlname Structure

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:

`length`

  Contains the length of the field name.

`data`

  Contains the actual field name.

#### 35.7.2.2. Retrieving a Result Set Using an SQLDA

The general steps to retrieve a query result set through an SQLDA are:

1. Declare an `sqlda_t` structure to receive the result set.

2. Execute `FETCH`/`EXECUTE`/`DESCRIBE` commands to process a query specifying the declared SQLDA.

3. Check the number of records in the result set by looking at `sqln`, a member of the `sqlda_t` structure.

4. Get the values of each column from `sqlvar[0]`, `sqlvar[1]`, etc., members of the `sqlda_t` structure.

5. Go to next row (`sqlda_t` structure) by following the `desc_next` pointer, a member of the `sqlda_t` structure.

6. 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;

        ...
    }

#### 35.7.2.3. Passing Query Parameters Using an SQLDA

The general steps to use an SQLDA to pass input parameters to a prepared query are:

1. Create a prepared query (prepared statement)

2. Declare a sqlda_t structure as an input SQLDA.

3. Allocate memory area (as sqlda_t structure) for the input SQLDA.

4. Set (copy) input values in the allocated memory.

5. 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);

#### 35.7.2.4. A Sample Application Using SQLDA

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 35.1.

Example 35.1. 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 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;
    }

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)
    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)

Prev | Up | Next
35.6. pgtypes Library | Home | 35.8. Error Handling