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PG-562: Histogram Ranges/Buckets are not correct.

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Added buckets for queries that take less than minimum histogram time
and one for the ones taking more than the max value specified.

Also, in case the buckets end up overlapping, on server start, an
error will be thrown informing the user of this issue and requesting
a rectification.

Refactored the code to consolidate the calculations in a single
function.
This commit is contained in:
Hamid Akhtar
2023-01-19 05:39:37 +05:00
committed by Muhammad Usama
parent 1286427445
commit 209f370cef
8 changed files with 109 additions and 92 deletions
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127
pg_stat_monitor.c
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@@ -93,6 +93,7 @@ static struct rusage rusage_end;
static char *pgss_explain(QueryDesc *queryDesc);
static void extract_query_comments(const char *query, char *comments, size_t max_len);
static void histogram_bucket_timings(int index, int64 *b_start, int64 *b_end);
static int get_histogram_bucket(double q_time);
static bool IsSystemInitialized(void);
static double time_diff(struct timeval end, struct timeval start);
@@ -254,6 +255,24 @@ _PG_init(void)
/* Inilize the GUC variables */
init_guc();
/* Validate histogram values and in case of failure, revert to defaults */
{
int64 b1_start;
int64 b2_start;
int64 b1_end;
int64 b2_end;
if (PGSM_HISTOGRAM_BUCKETS_USER >= 2)
{
histogram_bucket_timings(1, &b1_start, &b1_end);
histogram_bucket_timings(2, &b2_start, &b2_end);
/* Checking if histograms buckets overlap */
if (b2_start < b1_end || b2_start == b2_end)
elog(ERROR, "pg_stat_monitor: Histogram buckets are overlapping, please fix histogram configuration.");
}
}
#if PG_VERSION_NUM >= 140000
/*
@@ -3238,65 +3257,99 @@ unpack_sql_state(int sql_state)
return buf;
}
/*
* Given an index, return the histogram start and end times.
*/
static void
histogram_bucket_timings(int index, int64 *b_start, int64 *b_end)
{
double q_min = PGSM_HISTOGRAM_MIN;
double q_max = PGSM_HISTOGRAM_MAX;
int b_count = PGSM_HISTOGRAM_BUCKETS;
int b_count_user = PGSM_HISTOGRAM_BUCKETS_USER;
double bucket_size;
/*
* We must not skip any queries that fall outside the user defined
* histogram buckets. So capturing min/max outliers.
*/
if (index == 0 && q_min > 0)
{
*b_start = 0;
*b_end = q_min;
return;
}
else if (index == (b_count - 1))
{
*b_start = q_max;
*b_end = -1;
return;
}
/*
* Equisized logrithmic values will yield exponential values as required.
* For calculating logrithmic value, we MUST use the number of bucket provided
* by the user.
*/
bucket_size = log(q_max - q_min) / (double) b_count_user;
/* Can't do exp(0) as that returns 1. So handling the case of first entry specifically */
*b_start = q_min + ((index == 0 || (index == 1 && q_min > 0)) ? 0 : exp(bucket_size * (index - 1)));
*b_end = q_min + exp(bucket_size * index);
}
/*
* Get the histogram bucket index for a given query time.
*/
static int
get_histogram_bucket(double q_time)
{
double q_min = PGSM_HISTOGRAM_MIN;
double q_max = PGSM_HISTOGRAM_MAX;
int64 b_start;
int64 b_end;
int b_count = PGSM_HISTOGRAM_BUCKETS;
int index = 0;
double b_max;
double b_min;
double bucket_size;
q_time -= q_min;
b_max = log(q_max - q_min);
b_min = 0;
bucket_size = (b_max - b_min) / (double) b_count;
for (index = 1; index <= b_count; index++)
for (index = 0; index < b_count; index++)
{
int64 b_start = (index == 1) ? 0 : exp(bucket_size * (index - 1));
int64 b_end = exp(bucket_size * index);
histogram_bucket_timings(index, &b_start, &b_end);
if ((index == 1 && q_time < b_start)
|| (q_time >= b_start && q_time <= b_end)
|| (index == b_count && q_time > b_end))
{
return index - 1;
}
if (q_time >= b_start && q_time <= b_end)
return index;
}
return 0;
/*
* So haven't found a histogram bucket for this query. That's only possible for the
* last bucket as its end time is less than 0.
*/
return (b_count - 1);
}
/*
* Get the timings of the histogram as a single string. The last bucket
* has ellipses as the end value indication infinity.
*/
Datum
get_histogram_timings(PG_FUNCTION_ARGS)
{
double q_min = PGSM_HISTOGRAM_MIN;
double q_max = PGSM_HISTOGRAM_MAX;
int64 b_start;
int64 b_end;
int b_count = PGSM_HISTOGRAM_BUCKETS;
int index = 0;
double b_max;
double b_min;
double bucket_size;
bool first = true;
char *tmp_str = palloc0(MAX_STRING_LEN);
char *text_str = palloc0(MAX_STRING_LEN);
b_max = log(q_max - q_min);
b_min = 0;
bucket_size = (b_max - b_min) / (double) b_count;
for (index = 1; index <= b_count; index++)
for (index = 0; index < b_count; index++)
{
int64 b_start = (index == 1) ? 0 : exp(bucket_size * (index - 1));
int64 b_end = exp(bucket_size * index);
histogram_bucket_timings(index, &b_start, &b_end);
if (first)
if (index == 0)
{
snprintf(text_str, MAX_STRING_LEN, "(%ld - %ld)}", b_start, b_end);
first = false;
}
else if (index == (b_count - 1))
{
snprintf(tmp_str, MAX_STRING_LEN, "%s, (%ld - ...)}", text_str, b_start);
snprintf(text_str, MAX_STRING_LEN, "%s", tmp_str);
}
else
{
@@ -3304,7 +3357,9 @@ get_histogram_timings(PG_FUNCTION_ARGS)
snprintf(text_str, MAX_STRING_LEN, "%s", tmp_str);
}
}
pfree(tmp_str);
return CStringGetTextDatum(text_str);
}