1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!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>36.7. 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 Vsnapshot" /><link rel="prev" href="ecpg-pgtypes.html" title="36.6. pgtypes Library" /><link rel="next" href="ecpg-errors.html" title="36.8. Error Handling" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">36.7. Using Descriptor Areas</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 36. <span class="application">ECPG</span> — Embedded <acronym class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-errors.html" title="36.8. Error Handling">Next</a></td></tr></table><hr /></div><div class="sect1" id="ECPG-DESCRIPTORS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">36.7. 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">36.7.1. Named SQL Descriptor Areas</a></span></dt><dt><span class="sect2"><a href="ecpg-descriptors.html#ECPG-SQLDA-DESCRIPTORS">36.7.2. SQLDA Descriptor Areas</a></span></dt></dl></div><p>
An SQL descriptor area is a more sophisticated method for processing
the result of a <code class="command">SELECT</code>, <code class="command">FETCH</code> or
a <code class="command">DESCRIBE</code> 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" id="ECPG-NAMED-DESCRIPTORS"><div class="titlepage"><div><div><h3 class="title">36.7.1. Named SQL Descriptor Areas</h3></div></div></div><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 <em class="replaceable"><code>identifier</code></em>;
</pre><p>
The identifier serves as the <span class="quote">“<span class="quote">variable name</span>”</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 <em class="replaceable"><code>identifier</code></em>;
</pre><p>
</p><p>
To use a descriptor area, specify it as the storage target in an
<code class="literal">INTO</code> 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 <code class="command">DESCRIBE</code>
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 <code class="literal">SQL</code> keyword was optional,
so using <code class="literal">DESCRIPTOR</code> and <code class="literal">SQL DESCRIPTOR</code>
produced named SQL Descriptor Areas. Now it is mandatory, omitting
the <code class="literal">SQL</code> keyword produces SQLDA Descriptor Areas,
see <a class="xref" href="ecpg-descriptors.html#ECPG-SQLDA-DESCRIPTORS" title="36.7.2. SQLDA Descriptor Areas">Section 36.7.2</a>.
</p><p>
In <code class="command">DESCRIBE</code> and <code class="command">FETCH</code> statements,
the <code class="literal">INTO</code> and <code class="literal">USING</code> 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 <em class="replaceable"><code>name</code></em> :<em class="replaceable"><code>hostvar</code></em> = <em class="replaceable"><code>field</code></em>;
</pre><p>
Currently, there is only one header field defined:
<em class="replaceable"><code>COUNT</code></em>, 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 <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>;
</pre><p>
<em class="replaceable"><code>num</code></em> can be a literal integer or a host
variable containing an integer. Possible fields are:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">CARDINALITY</code> (integer)</span></dt><dd><p>
number of rows in the result set
</p></dd><dt><span class="term"><code class="literal">DATA</code></span></dt><dd><p>
actual data item (therefore, the data type of this field
depends on the query)
</p></dd><dt><span class="term"><code class="literal">DATETIME_INTERVAL_CODE</code> (integer)</span></dt><dd><p>
When <code class="literal">TYPE</code> is <code class="literal">9</code>,
<code class="literal">DATETIME_INTERVAL_CODE</code> will have a value of
<code class="literal">1</code> for <code class="literal">DATE</code>,
<code class="literal">2</code> for <code class="literal">TIME</code>,
<code class="literal">3</code> for <code class="literal">TIMESTAMP</code>,
<code class="literal">4</code> for <code class="literal">TIME WITH TIME ZONE</code>, or
<code class="literal">5</code> for <code class="literal">TIMESTAMP WITH TIME ZONE</code>.
</p></dd><dt><span class="term"><code class="literal">DATETIME_INTERVAL_PRECISION</code> (integer)</span></dt><dd><p>
not implemented
</p></dd><dt><span class="term"><code class="literal">INDICATOR</code> (integer)</span></dt><dd><p>
the indicator (indicating a null value or a value truncation)
</p></dd><dt><span class="term"><code class="literal">KEY_MEMBER</code> (integer)</span></dt><dd><p>
not implemented
</p></dd><dt><span class="term"><code class="literal">LENGTH</code> (integer)</span></dt><dd><p>
length of the datum in characters
</p></dd><dt><span class="term"><code class="literal">NAME</code> (string)</span></dt><dd><p>
name of the column
</p></dd><dt><span class="term"><code class="literal">NULLABLE</code> (integer)</span></dt><dd><p>
not implemented
</p></dd><dt><span class="term"><code class="literal">OCTET_LENGTH</code> (integer)</span></dt><dd><p>
length of the character representation of the datum in bytes
</p></dd><dt><span class="term"><code class="literal">PRECISION</code> (integer)</span></dt><dd><p>
precision (for type <code class="type">numeric</code>)
</p></dd><dt><span class="term"><code class="literal">RETURNED_LENGTH</code> (integer)</span></dt><dd><p>
length of the datum in characters
</p></dd><dt><span class="term"><code class="literal">RETURNED_OCTET_LENGTH</code> (integer)</span></dt><dd><p>
length of the character representation of the datum in bytes
</p></dd><dt><span class="term"><code class="literal">SCALE</code> (integer)</span></dt><dd><p>
scale (for type <code class="type">numeric</code>)
</p></dd><dt><span class="term"><code class="literal">TYPE</code> (integer)</span></dt><dd><p>
numeric code of the data type of the column
</p></dd></dl></div><p>
</p><p>
In <code class="command">EXECUTE</code>, <code class="command">DECLARE</code> and <code class="command">OPEN</code>
statements, the effect of the <code class="literal">INTO</code> and <code class="literal">USING</code>
keywords are different. A Descriptor Area can also be manually built to
provide the input parameters for a query or a cursor and
<code class="literal">USING SQL DESCRIPTOR <em class="replaceable"><code>name</code></em></code>
is the way to pass the input parameters into a parameterized query. The statement
to build a named SQL Descriptor Area is below:
</p><pre class="programlisting">
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>;
</pre><p>
</p><p>
PostgreSQL supports retrieving more that one record in one <code class="command">FETCH</code>
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" id="ECPG-SQLDA-DESCRIPTORS"><div class="titlepage"><div><div><h3 class="title">36.7.2. SQLDA Descriptor Areas</h3></div></div></div><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 <code class="literal">SQL</code> keyword is omitted. The paragraphs about
the use cases of the <code class="literal">INTO</code> and <code class="literal">USING</code>
keywords in <a class="xref" href="ecpg-descriptors.html#ECPG-NAMED-DESCRIPTORS" title="36.7.1. Named SQL Descriptor Areas">Section 36.7.1</a> also apply here with an addition.
In a <code class="command">DESCRIBE</code> statement the <code class="literal">DESCRIPTOR</code>
keyword can be completely omitted if the <code class="literal">INTO</code> 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 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">36.7.2.1. SQLDA Data Structure</h4></div></div></div><p>
SQLDA uses three data structure
types: <code class="type">sqlda_t</code>, <code class="type">sqlvar_t</code>,
and <code class="type">struct sqlname</code>.
</p><div class="tip"><h3 class="title">Tip</h3><p>
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></div><div class="sect4" id="ECPG-SQLDA-SQLDA"><div class="titlepage"><div><div><h5 class="title">36.7.2.1.1. sqlda_t Structure</h5></div></div></div><p>
The structure type <code class="type">sqlda_t</code> is the type of the
actual SQLDA. It holds one record. And two or
more <code class="type">sqlda_t</code> structures can be connected in a
linked list with the pointer in
the <code class="structfield">desc_next</code> field, thus
representing an ordered collection of rows. So, when two or
more rows are fetched, the application can read them by
following the <code class="structfield">desc_next</code> pointer in
each <code class="type">sqlda_t</code> node.
</p><p>
The definition of <code class="type">sqlda_t</code> 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><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">sqldaid</code></span></dt><dd><p>
It contains the literal string <code class="literal">"SQLDA "</code>.
</p></dd><dt><span class="term"><code class="literal">sqldabc</code></span></dt><dd><p>
It contains the size of the allocated space in bytes.
</p></dd><dt><span class="term"><code class="literal">sqln</code></span></dt><dd><p>
It contains the number of input parameters for a parameterized query in
case it's passed into <code class="command">OPEN</code>, <code class="command">DECLARE</code> or
<code class="command">EXECUTE</code> statements using the <code class="literal">USING</code>
keyword. In case it's used as output of <code class="command">SELECT</code>,
<code class="command">EXECUTE</code> or <code class="command">FETCH</code> statements,
its value is the same as <code class="literal">sqld</code>
statement
</p></dd><dt><span class="term"><code class="literal">sqld</code></span></dt><dd><p>
It contains the number of fields in a result set.
</p></dd><dt><span class="term"><code class="literal">desc_next</code></span></dt><dd><p>
If the query returns more than one record, multiple linked
SQLDA structures are returned, and <code class="literal">desc_next</code> holds
a pointer to the next entry in the list.
</p></dd><dt><span class="term"><code class="literal">sqlvar</code></span></dt><dd><p>
This is the array of the columns in the result set.
</p></dd></dl></div><p>
</p></div><div class="sect4" id="ECPG-SQLDA-SQLVAR"><div class="titlepage"><div><div><h5 class="title">36.7.2.1.2. sqlvar_t Structure</h5></div></div></div><p>
The structure type <code class="type">sqlvar_t</code> 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><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">sqltype</code></span></dt><dd><p>
Contains the type identifier of the field. For values,
see <code class="literal">enum ECPGttype</code> in <code class="literal">ecpgtype.h</code>.
</p></dd><dt><span class="term"><code class="literal">sqllen</code></span></dt><dd><p>
Contains the binary length of the field. e.g., 4 bytes for <code class="type">ECPGt_int</code>.
</p></dd><dt><span class="term"><code class="literal">sqldata</code></span></dt><dd><p>
Points to the data. The format of the data is described
in <a class="xref" href="ecpg-variables.html#ECPG-VARIABLES-TYPE-MAPPING" title="36.4.4. Type Mapping">Section 36.4.4</a>.
</p></dd><dt><span class="term"><code class="literal">sqlind</code></span></dt><dd><p>
Points to the null indicator. 0 means not null, -1 means
null.
</p></dd><dt><span class="term"><code class="literal">sqlname</code></span></dt><dd><p>
The name of the field.
</p></dd></dl></div><p>
</p></div><div class="sect4" id="ECPG-SQLDA-SQLNAME"><div class="titlepage"><div><div><h5 class="title">36.7.2.1.3. struct sqlname Structure</h5></div></div></div><p>
A <code class="type">struct sqlname</code> structure holds a column name. It
is used as a member of the <code class="type">sqlvar_t</code> 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><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">length</code></span></dt><dd><p>
Contains the length of the field name.
</p></dd><dt><span class="term"><code class="literal">data</code></span></dt><dd><p>
Contains the actual field name.
</p></dd></dl></div><p>
</p></div></div><div class="sect3" id="ECPG-SQLDA-OUTPUT"><div class="titlepage"><div><div><h4 class="title">36.7.2.2. Retrieving a Result Set Using an SQLDA</h4></div></div></div><div class="procedure"><p>
The general steps to retrieve a query result set through an
SQLDA are:
</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>
Here is an example retrieving a result set through an SQLDA.
</p><p>
First, declare a <code class="type">sqlda_t</code> 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 <code class="command">FETCH</code> 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
(<code class="type">sqlvar_t</code> 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 <code class="structfield">sqltype</code> value,
a member of the <code class="type">sqlvar_t</code> structure. Then, switch
to an appropriate way, depending on the column type, to copy
data from the <code class="structfield">sqlvar</code> 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" id="ECPG-SQLDA-INPUT"><div class="titlepage"><div><div><h4 class="title">36.7.2.3. Passing Query Parameters Using an SQLDA</h4></div></div></div><div class="procedure"><p>
The general steps to use an SQLDA to pass input
parameters to a prepared query are:
</p><ol class="procedure" type="1"><li class="step"><p>Create a prepared query (prepared statement)</p></li><li class="step"><p>Declare an 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 <code class="structfield">sqln</code>, a member variable of
the <code class="type">sqlda_t</code> 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
<code class="literal">sqlvar[]</code> 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 <code class="literal">"postgres"</code>, having a string type,
and <code class="literal">1</code>, 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" id="ECPG-SQLDA-EXAMPLE"><div class="titlepage"><div><div><h4 class="title">36.7.2.4. A Sample Application Using SQLDA</h4></div></div></div><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 <code class="literal">postgres</code>, and OID <code class="literal">1</code>).
</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 SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
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 <code class="literal">sqln</code>. Store
type, value, and value length into <code class="literal">sqltype</code>,
<code class="literal">sqldata</code>, and <code class="literal">sqllen</code> in the
<code class="literal">sqlvar</code> 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 <code class="command">FETCH</code> 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 <code class="type">sqlda_t</code> 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 <code class="structfield">sqld</code>, the actual data of the first
column is stored in <code class="literal">sqlvar[0]</code>, both members of
the <code class="type">sqlda_t</code> 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 <code class="varname">v</code>.
Copy every datum into host variables, looking
at <code class="literal">v.sqltype</code> 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 class="xref" href="ecpg-descriptors.html#ECPG-SQLDA-EXAMPLE-EXAMPLE" title="Example 36.1. Example SQLDA Program">Example 36.1</a>.
</p><div class="example" id="ECPG-SQLDA-EXAMPLE-EXAMPLE"><p class="title"><strong>Example 36.1. Example SQLDA Program</strong></p><div class="example-contents"><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 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;
}
</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)
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)
</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" title="36.6. pgtypes Library">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-errors.html" title="36.8. Error Handling">Next</a></td></tr><tr><td width="40%" align="left" valign="top">36.6. pgtypes Library </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 36.8. Error Handling</td></tr></table></div></body></html>
|
