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Add \gdesc psql command.
This command acts somewhat like \g, but instead of executing the query buffer, it merely prints a description of the columns that the query result would have. (Of course, this still requires parsing the query; if parse analysis fails, you get an error anyway.) We accomplish this using an unnamed prepared statement, which should be invisible to psql users. Pavel Stehule, reviewed by Fabien Coelho Discussion: https://postgr.es/m/CAFj8pRBhYVvO34FU=EKb=nAF5t3b++krKt1FneCmR0kuF5m-QA@mail.gmail.com
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‎doc/src/sgml/ref/psql-ref.sgml

Lines changed: 19 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1949,6 +1949,25 @@ Tue Oct 26 21:40:57 CEST 1999
19491949
</varlistentry>
19501950

19511951

1952+
<varlistentry>
1953+
<term><literal>\gdesc</literal></term>
1954+
1955+
<listitem>
1956+
<para>
1957+
Shows the description (that is, the column names and data types)
1958+
of the result of the current query buffer. The query is not
1959+
actually executed; however, if it contains some type of syntax
1960+
error, that error will be reported in the normal way.
1961+
</para>
1962+
1963+
<para>
1964+
If the current query buffer is empty, the most recently sent query
1965+
is described instead.
1966+
</para>
1967+
</listitem>
1968+
</varlistentry>
1969+
1970+
19521971
<varlistentry>
19531972
<term><literal>\gexec</literal></term>
19541973

‎src/bin/psql/command.c

Lines changed: 20 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -88,6 +88,7 @@ static backslashResult exec_command_errverbose(PsqlScanState scan_state, bool ac
8888
static backslashResult exec_command_f(PsqlScanState scan_state, bool active_branch);
8989
static backslashResult exec_command_g(PsqlScanState scan_state, bool active_branch,
9090
const char *cmd);
91+
static backslashResult exec_command_gdesc(PsqlScanState scan_state, bool active_branch);
9192
static backslashResult exec_command_gexec(PsqlScanState scan_state, bool active_branch);
9293
static backslashResult exec_command_gset(PsqlScanState scan_state, bool active_branch);
9394
static backslashResult exec_command_help(PsqlScanState scan_state, bool active_branch);
@@ -337,6 +338,8 @@ exec_command(const char *cmd,
337338
status = exec_command_f(scan_state, active_branch);
338339
else if (strcmp(cmd, "g") == 0 || strcmp(cmd, "gx") == 0)
339340
status = exec_command_g(scan_state, active_branch, cmd);
341+
else if (strcmp(cmd, "gdesc") == 0)
342+
status = exec_command_gdesc(scan_state, active_branch);
340343
else if (strcmp(cmd, "gexec") == 0)
341344
status = exec_command_gexec(scan_state, active_branch);
342345
else if (strcmp(cmd, "gset") == 0)
@@ -1330,6 +1333,23 @@ exec_command_g(PsqlScanState scan_state, bool active_branch, const char *cmd)
13301333
return status;
13311334
}
13321335

1336+
/*
1337+
* \gdesc -- describe query result
1338+
*/
1339+
static backslashResult
1340+
exec_command_gdesc(PsqlScanState scan_state, bool active_branch)
1341+
{
1342+
backslashResult status = PSQL_CMD_SKIP_LINE;
1343+
1344+
if (active_branch)
1345+
{
1346+
pset.gdesc_flag = true;
1347+
status = PSQL_CMD_SEND;
1348+
}
1349+
1350+
return status;
1351+
}
1352+
13331353
/*
13341354
* \gexec -- send query and execute each field of result
13351355
*/

‎src/bin/psql/common.c

Lines changed: 128 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -29,6 +29,7 @@
2929
#include "fe_utils/mbprint.h"
3030

3131

32+
static bool DescribeQuery(const char *query, double *elapsed_msec);
3233
static bool ExecQueryUsingCursor(const char *query, double *elapsed_msec);
3334
static bool command_no_begin(const char *query);
3435
static bool is_select_command(const char *query);
@@ -1323,8 +1324,15 @@ SendQuery(const char *query)
13231324
}
13241325
}
13251326

1326-
if (pset.fetch_count <= 0 || pset.gexec_flag ||
1327-
pset.crosstab_flag || !is_select_command(query))
1327+
if (pset.gdesc_flag)
1328+
{
1329+
/* Describe query's result columns, without executing it */
1330+
OK = DescribeQuery(query, &elapsed_msec);
1331+
ResetCancelConn();
1332+
results = NULL; /* PQclear(NULL) does nothing */
1333+
}
1334+
else if (pset.fetch_count <= 0 || pset.gexec_flag ||
1335+
pset.crosstab_flag || !is_select_command(query))
13281336
{
13291337
/* Default fetch-it-all-and-print mode */
13301338
instr_time before,
@@ -1467,6 +1475,9 @@ SendQuery(const char *query)
14671475
pset.gset_prefix = NULL;
14681476
}
14691477

1478+
/* reset \gdesc trigger */
1479+
pset.gdesc_flag = false;
1480+
14701481
/* reset \gexec trigger */
14711482
pset.gexec_flag = false;
14721483

@@ -1482,6 +1493,118 @@ SendQuery(const char *query)
14821493
}
14831494

14841495

1496+
/*
1497+
* DescribeQuery: describe the result columns of a query, without executing it
1498+
*
1499+
* Returns true if the operation executed successfully, false otherwise.
1500+
*
1501+
* If pset.timing is on, total query time (exclusive of result-printing) is
1502+
* stored into *elapsed_msec.
1503+
*/
1504+
static bool
1505+
DescribeQuery(const char *query, double *elapsed_msec)
1506+
{
1507+
PGresult *results;
1508+
bool OK;
1509+
instr_time before,
1510+
after;
1511+
1512+
*elapsed_msec = 0;
1513+
1514+
if (pset.timing)
1515+
INSTR_TIME_SET_CURRENT(before);
1516+
1517+
/*
1518+
* To parse the query but not execute it, we prepare it, using the unnamed
1519+
* prepared statement. This is invisible to psql users, since there's no
1520+
* way to access the unnamed prepared statement from psql user space. The
1521+
* next Parse or Query protocol message would overwrite the statement
1522+
* anyway. (So there's no great need to clear it when done, which is a
1523+
* good thing because libpq provides no easy way to do that.)
1524+
*/
1525+
results = PQprepare(pset.db, "", query, 0, NULL);
1526+
if (PQresultStatus(results) != PGRES_COMMAND_OK)
1527+
{
1528+
psql_error("%s", PQerrorMessage(pset.db));
1529+
ClearOrSaveResult(results);
1530+
return false;
1531+
}
1532+
PQclear(results);
1533+
1534+
results = PQdescribePrepared(pset.db, "");
1535+
OK = AcceptResult(results) &&
1536+
(PQresultStatus(results) == PGRES_COMMAND_OK);
1537+
if (OK && results)
1538+
{
1539+
if (PQnfields(results) > 0)
1540+
{
1541+
PQExpBufferData buf;
1542+
int i;
1543+
1544+
initPQExpBuffer(&buf);
1545+
1546+
printfPQExpBuffer(&buf,
1547+
"SELECT name AS \"%s\", pg_catalog.format_type(tp, tpm) AS \"%s\"\n"
1548+
"FROM (VALUES ",
1549+
gettext_noop("Column"),
1550+
gettext_noop("Type"));
1551+
1552+
for (i = 0; i < PQnfields(results); i++)
1553+
{
1554+
const char *name;
1555+
char *escname;
1556+
1557+
if (i > 0)
1558+
appendPQExpBufferStr(&buf, ",");
1559+
1560+
name = PQfname(results, i);
1561+
escname = PQescapeLiteral(pset.db, name, strlen(name));
1562+
1563+
if (escname == NULL)
1564+
{
1565+
psql_error("%s", PQerrorMessage(pset.db));
1566+
PQclear(results);
1567+
termPQExpBuffer(&buf);
1568+
return false;
1569+
}
1570+
1571+
appendPQExpBuffer(&buf, "(%s, '%u'::pg_catalog.oid, %d)",
1572+
escname,
1573+
PQftype(results, i),
1574+
PQfmod(results, i));
1575+
1576+
PQfreemem(escname);
1577+
}
1578+
1579+
appendPQExpBufferStr(&buf, ") s(name, tp, tpm)");
1580+
PQclear(results);
1581+
1582+
results = PQexec(pset.db, buf.data);
1583+
OK = AcceptResult(results);
1584+
1585+
if (pset.timing)
1586+
{
1587+
INSTR_TIME_SET_CURRENT(after);
1588+
INSTR_TIME_SUBTRACT(after, before);
1589+
*elapsed_msec += INSTR_TIME_GET_MILLISEC(after);
1590+
}
1591+
1592+
if (OK && results)
1593+
OK = PrintQueryResults(results);
1594+
1595+
termPQExpBuffer(&buf);
1596+
}
1597+
else
1598+
fprintf(pset.queryFout,
1599+
_("The command has no result, or the result has no columns.\n"));
1600+
}
1601+
1602+
ClearOrSaveResult(results);
1603+
1604+
return OK;
1605+
}
1606+
1607+
14851608
/*
14861609
* ExecQueryUsingCursor: run a SELECT-like query using a cursor
14871610
*
@@ -1627,7 +1750,9 @@ ExecQueryUsingCursor(const char *query, double *elapsed_msec)
16271750
break;
16281751
}
16291752

1630-
/* Note we do not deal with \gexec or \crosstabview modes here */
1753+
/*
1754+
* Note we do not deal with \gdesc, \gexec or \crosstabview modes here
1755+
*/
16311756

16321757
ntuples = PQntuples(results);
16331758

‎src/bin/psql/help.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -167,13 +167,14 @@ slashUsage(unsigned short int pager)
167167
* Use "psql --help=commands | wc" to count correctly. It's okay to count
168168
* the USE_READLINE line even in builds without that.
169169
*/
170-
output = PageOutput(122, pager ? &(pset.popt.topt) : NULL);
170+
output = PageOutput(125, pager ? &(pset.popt.topt) : NULL);
171171

172172
fprintf(output, _("General\n"));
173173
fprintf(output, _(" \\copyright show PostgreSQL usage and distribution terms\n"));
174174
fprintf(output, _(" \\crosstabview [COLUMNS] execute query and display results in crosstab\n"));
175175
fprintf(output, _(" \\errverbose show most recent error message at maximum verbosity\n"));
176176
fprintf(output, _(" \\g [FILE] or ; execute query (and send results to file or |pipe)\n"));
177+
fprintf(output, _(" \\gdesc describe result of query, without executing it\n"));
177178
fprintf(output, _(" \\gexec execute query, then execute each value in its result\n"));
178179
fprintf(output, _(" \\gset [PREFIX] execute query and store results in psql variables\n"));
179180
fprintf(output, _(" \\gx [FILE] as \\g, but forces expanded output mode\n"));

‎src/bin/psql/settings.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -93,7 +93,8 @@ typedef struct _psqlSettings
9393
char *gfname; /* one-shot file output argument for \g */
9494
bool g_expanded; /* one-shot expanded output requested via \gx */
9595
char *gset_prefix; /* one-shot prefix argument for \gset */
96-
bool gexec_flag; /* one-shot flag to execute query's results */
96+
bool gdesc_flag; /* one-shot request to describe query results */
97+
bool gexec_flag; /* one-shot request to execute query results */
9798
bool crosstab_flag; /* one-shot request to crosstab results */
9899
char *ctv_args[4]; /* \crosstabview arguments */
99100

‎src/bin/psql/tab-complete.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1433,7 +1433,7 @@ psql_completion(const char *text, int start, int end)
14331433
"\\e", "\\echo", "\\ef", "\\elif", "\\else", "\\encoding",
14341434
"\\endif", "\\errverbose", "\\ev",
14351435
"\\f",
1436-
"\\g", "\\gexec", "\\gset", "\\gx",
1436+
"\\g", "\\gdesc", "\\gexec", "\\gset", "\\gx",
14371437
"\\h", "\\help", "\\H",
14381438
"\\i", "\\if", "\\ir",
14391439
"\\l", "\\lo_import", "\\lo_export", "\\lo_list", "\\lo_unlink",

‎src/test/regress/expected/psql.out

Lines changed: 85 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -126,6 +126,91 @@ more than one row returned for \gset
126126
select 10 as test01, 20 as test02 from generate_series(1,0) \gset
127127
no rows returned for \gset
128128
\unset FETCH_COUNT
129+
-- \gdesc
130+
SELECT
131+
NULL AS zero,
132+
1 AS one,
133+
2.0 AS two,
134+
'three' AS three,
135+
$1 AS four,
136+
sin($2) as five,
137+
'foo'::varchar(4) as six,
138+
CURRENT_DATE AS now
139+
\gdesc
140+
Column | Type
141+
--------+----------------------
142+
zero | text
143+
one | integer
144+
two | numeric
145+
three | text
146+
four | text
147+
five | double precision
148+
six | character varying(4)
149+
now | date
150+
(8 rows)
151+
152+
-- should work with tuple-returning utilities, such as EXECUTE
153+
PREPARE test AS SELECT 1 AS first, 2 AS second;
154+
EXECUTE test \gdesc
155+
Column | Type
156+
--------+---------
157+
first | integer
158+
second | integer
159+
(2 rows)
160+
161+
EXPLAIN EXECUTE test \gdesc
162+
Column | Type
163+
------------+------
164+
QUERY PLAN | text
165+
(1 row)
166+
167+
-- should fail cleanly - syntax error
168+
SELECT 1 + \gdesc
169+
ERROR: syntax error at end of input
170+
LINE 1: SELECT 1 +
171+
^
172+
-- check behavior with empty results
173+
SELECT \gdesc
174+
The command has no result, or the result has no columns.
175+
CREATE TABLE bububu(a int) \gdesc
176+
The command has no result, or the result has no columns.
177+
-- subject command should not have executed
178+
TABLE bububu; -- fail
179+
ERROR: relation "bububu" does not exist
180+
LINE 1: TABLE bububu;
181+
^
182+
-- query buffer should remain unchanged
183+
SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty\name"
184+
\gdesc
185+
Column | Type
186+
------------+---------
187+
x | integer
188+
?column? | text
189+
y | integer
190+
dirty\name | boolean
191+
(4 rows)
192+
193+
\g
194+
x | ?column? | y | dirty\name
195+
---+----------+---+------------
196+
1 | Hello | 2 | t
197+
(1 row)
198+
199+
-- all on one line
200+
SELECT 3 AS x, 'Hello', 4 AS y, true AS "dirty\name" \gdesc \g
201+
Column | Type
202+
------------+---------
203+
x | integer
204+
?column? | text
205+
y | integer
206+
dirty\name | boolean
207+
(4 rows)
208+
209+
x | ?column? | y | dirty\name
210+
---+----------+---+------------
211+
3 | Hello | 4 | t
212+
(1 row)
213+
129214
-- \gexec
130215
create temporary table gexec_test(a int, b text, c date, d float);
131216
select format('create index on gexec_test(%I)', attname)

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