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Allow \watch queries to stop on minimum rows returned 

When running a repeat query with \watch in psql, it can be
helpful to be able to stop the watch process when the query
no longer returns the expected amount of rows.  An example
would be to watch for the presence of a certain event in
pg_stat_activity and stopping when the event is no longer
present, or to watch an index creation and stop when the
index is created.

This adds a min_rows=MIN parameter to \watch which can be
set to a non-negative integer, and the watch query will
stop executing when it returns less than MIN rows.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAKAnmmKStATuddYxP71L+p0DHtp9Rvjze3XRoy0Dyw67VQ45UA@mail.gmail.com
This commit is contained in:
Daniel Gustafsson 2023-08-29 11:30:11 +02:00
parent 95fff2abee
commit f347ec76e2
6 changed files with 72 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
doc/src/sgml/ref/psql-ref.sgml
View File

@ -3566,13 +3566,14 @@ testdb=&gt; <userinput>\setenv LESS -imx4F</userinput>
<varlistentry id="app-psql-meta-command-watch">
<term><literal>\watch [ i[nterval]=<replaceable class="parameter">seconds</replaceable> ] [ c[ount]=<replaceable class="parameter">times</replaceable> ] [ <replaceable class="parameter">seconds</replaceable> ]</literal></term>
<term><literal>\watch [ i[nterval]=<replaceable class="parameter">seconds</replaceable> ] [ c[ount]=<replaceable class="parameter">times</replaceable> ] [ m[in_rows]=<replaceable class="parameter">rows</replaceable> ] [ <replaceable class="parameter">seconds</replaceable> ]</literal></term>
<listitem>
<para>
Repeatedly execute the current query buffer (as <literal>\g</literal> does)
until interrupted, or the query fails, or the execution count limit
(if given) is reached. Wait the specified number of
seconds (default 2) between executions. For backwards compatibility,
(if given) is reached, or the query no longer returns the minimum number
of rows. Wait the specified number of seconds (default 2) between executions.
For backwards compatibility,
<replaceable class="parameter">seconds</replaceable> can be specified
with or without an <literal>interval=</literal> prefix.
Each query result is

32
src/bin/psql/command.c
View File

@ -162,7 +162,7 @@ static bool do_connect(enum trivalue reuse_previous_specification,
static bool do_edit(const char *filename_arg, PQExpBuffer query_buf,
int lineno, bool discard_on_quit, bool *edited);
static bool do_shell(const char *command);
static bool do_watch(PQExpBuffer query_buf, double sleep, int iter);
static bool do_watch(PQExpBuffer query_buf, double sleep, int iter, int min_rows);
static bool lookup_object_oid(EditableObjectType obj_type, const char *desc,
Oid *obj_oid);
static bool get_create_object_cmd(EditableObjectType obj_type, Oid oid,
@ -2775,13 +2775,15 @@ exec_command_watch(PsqlScanState scan_state, bool active_branch,
{
bool have_sleep = false;
bool have_iter = false;
bool have_min_rows = false;
double sleep = 2;
int iter = 0;
int min_rows = 0;
/*
* Parse arguments. We allow either an unlabeled interval or
* "name=value", where name is from the set ('i', 'interval', 'c',
* 'count').
* 'count', 'm', 'min_rows').
*/
while (success)
{
@ -2838,6 +2840,26 @@ exec_command_watch(PsqlScanState scan_state, bool active_branch,
}
}
}
else if (strncmp("m=", opt, strlen("m=")) == 0 ||
strncmp("min_rows=", opt, strlen("min_rows=")) == 0)
{
if (have_min_rows)
{
pg_log_error("\\watch: minimum row count specified more than once");
success = false;
}
else
{
have_min_rows = true;
errno = 0;
min_rows = strtoint(valptr, &opt_end, 10);
if (min_rows <= 0 || *opt_end || errno == ERANGE)
{
pg_log_error("\\watch: incorrect minimum row count \"%s\"", valptr);
success = false;
}
}
}
else
{
pg_log_error("\\watch: unrecognized parameter \"%s\"", opt);
@ -2874,7 +2896,7 @@ exec_command_watch(PsqlScanState scan_state, bool active_branch,
/* If query_buf is empty, recall and execute previous query */
(void) copy_previous_query(query_buf, previous_buf);
success = do_watch(query_buf, sleep, iter);
success = do_watch(query_buf, sleep, iter, min_rows);
}
/* Reset the query buffer as though for \r */
@ -5144,7 +5166,7 @@ do_shell(const char *command)
* onto a bunch of exec_command's variables to silence stupider compilers.
*/
static bool
do_watch(PQExpBuffer query_buf, double sleep, int iter)
do_watch(PQExpBuffer query_buf, double sleep, int iter, int min_rows)
{
long sleep_ms = (long) (sleep * 1000);
printQueryOpt myopt = pset.popt;
@ -5274,7 +5296,7 @@ do_watch(PQExpBuffer query_buf, double sleep, int iter)
myopt.title = title;
/* Run the query and print out the result */
res = PSQLexecWatch(query_buf->data, &myopt, pagerpipe);
res = PSQLexecWatch(query_buf->data, &myopt, pagerpipe, min_rows);
/*
* PSQLexecWatch handles the case where we can no longer repeat the

19
src/bin/psql/common.c
View File

@ -36,6 +36,7 @@ static int ExecQueryAndProcessResults(const char *query,
double *elapsed_msec,
bool *svpt_gone_p,
bool is_watch,
int min_rows,
const printQueryOpt *opt,
FILE *printQueryFout);
static bool command_no_begin(const char *query);
@ -632,7 +633,7 @@ PSQLexec(const char *query)
* e.g., because of the interrupt, -1 on error.
*/
int
PSQLexecWatch(const char *query, const printQueryOpt *opt, FILE *printQueryFout)
PSQLexecWatch(const char *query, const printQueryOpt *opt, FILE *printQueryFout, int min_rows)
{
bool timing = pset.timing;
double elapsed_msec = 0;
@ -646,7 +647,7 @@ PSQLexecWatch(const char *query, const printQueryOpt *opt, FILE *printQueryFout)
SetCancelConn(pset.db);
res = ExecQueryAndProcessResults(query, &elapsed_msec, NULL, true, opt, printQueryFout);
res = ExecQueryAndProcessResults(query, &elapsed_msec, NULL, true, min_rows, opt, printQueryFout);
ResetCancelConn();
@ -1134,7 +1135,7 @@ SendQuery(const char *query)
pset.crosstab_flag || !is_select_command(query))
{
/* Default fetch-it-all-and-print mode */
OK = (ExecQueryAndProcessResults(query, &elapsed_msec, &svpt_gone, false, NULL, NULL) > 0);
OK = (ExecQueryAndProcessResults(query, &elapsed_msec, &svpt_gone, false, 0, NULL, NULL) > 0);
}
else
{
@ -1415,11 +1416,12 @@ DescribeQuery(const char *query, double *elapsed_msec)
static int
ExecQueryAndProcessResults(const char *query,
double *elapsed_msec, bool *svpt_gone_p,
bool is_watch,
bool is_watch, int min_rows,
const printQueryOpt *opt, FILE *printQueryFout)
{
bool timing = pset.timing;
bool success;
bool return_early = false;
instr_time before,
after;
PGresult *result;
@ -1461,6 +1463,10 @@ ExecQueryAndProcessResults(const char *query,
/* first result */
result = PQgetResult(pset.db);
if (min_rows > 0 && PQntuples(result) < min_rows)
{
return_early = true;
}
while (result != NULL)
{
@ -1683,7 +1689,10 @@ ExecQueryAndProcessResults(const char *query,
if (!CheckConnection())
return -1;
return cancel_pressed ? 0 : success ? 1 : -1;
if (cancel_pressed || return_early)
return 0;
return success ? 1 : -1;
}

2
src/bin/psql/common.h
View File

@ -32,7 +32,7 @@ extern void psql_setup_cancel_handler(void);
extern void SetShellResultVariables(int wait_result);
extern PGresult *PSQLexec(const char *query);
extern int PSQLexecWatch(const char *query, const printQueryOpt *opt, FILE *printQueryFout);
extern int PSQLexecWatch(const char *query, const printQueryOpt *opt, FILE *printQueryFout, int min_rows);
extern bool SendQuery(const char *query);

4
src/bin/psql/help.c
View File

@ -200,7 +200,9 @@ slashUsage(unsigned short int pager)
HELP0(" \\gset [PREFIX] execute query and store result in psql variables\n");
HELP0(" \\gx [(OPTIONS)] [FILE] as \\g, but forces expanded output mode\n");
HELP0(" \\q quit psql\n");
HELP0(" \\watch [[i=]SEC] [c=N] execute query every SEC seconds, up to N times\n");
HELP0(" \\watch [[i=]SEC] [c=N] [m=MIN]\n");
HELP0(" execute query every SEC seconds, up to N times\n");
HELP0(" stop if less than MIN rows are returned\n");
HELP0("\n");
HELP0("Help\n");

23
src/bin/psql/t/001_basic.pl
View File

@ -355,6 +355,29 @@ psql_like(
psql_like($node, sprintf('SELECT 1 \watch c=3 i=%g', 0.01),
qr/1\n1\n1/, '\watch with 3 iterations');
# Check \watch minimum row count
psql_fails_like(
$node,
'SELECT 3 \watch m=x',
qr/incorrect minimum row count/,
'\watch, invalid minimum row setting');
psql_fails_like(
$node,
'SELECT 3 \watch m=1 min_rows=2',
qr/minimum row count specified more than once/,
'\watch, minimum rows is specified more than once');
psql_like(
$node,
q{with x as (
select now()-backend_start AS howlong
from pg_stat_activity
where pid = pg_backend_pid()
) select 123 from x where howlong < '2 seconds' \watch i=0.5 m=2},
qr/^123$/,
'\watch, 2 minimum rows');
# Check \watch errors
psql_fails_like(
$node,