PG -156: replace query placeholders with actual arguments for prepared statements #481
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Codecov ReportAttention: Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #481 +/- ##
==========================================
+ Coverage 84.88% 85.38% +0.50%
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Files 3 3
Lines 1297 1362 +65
Branches 198 208 +10
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+ Hits 1101 1163 +62
+ Misses 98 97 -1
- Partials 98 102 +4 ☔ View full report in Codecov by Sentry. |
artemgavrilov
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Added support for extracting query arguments for prepared statements when `pg_stat_monitor.pgsm_normalized_query` is off. Previously pg_stat_monitor was unable to extract the arguments for prepared statements, thus leaving queries with placeholders $1 .. $N instead of the actual arguments.
Instead of copying original query text byte by byte, copy data between
query placeholders in chunks, example:
`INSERT INTO foo(a, b, c) VALUES('test', 100, 'test again)'`
Would result in normalized query:
`INSERT INTO foo(a, b, c) VALUES($1, $2, $3)`
The original patch would copy the parts between placeholders byte by
byte, e.g. `INSERT INTO foo(a, b, c) VALUES(`, instead we can copy this
whole block at once, 1 function call and maybe 1 buffer re-allocation
per call.
Also make use of `appendBinaryStringInfo` to avoid calculating string
length as we have this info already.
Avoid allocating an array of strings for extracting query argument values, instead append the current parameter value directly in the buffer used to store the denormalized query. This avoids not only unnecessary memory allocations, but also copying data between temporary memory and the buffer.
This commit introduces a little optimization along with a feature, it stores the query in denormalized form only under the circumstances below: - The psgm_normalized_query GUC is disabled (off). - The query is seem for the first time, or the query total execution time exceeds the mean execution time calculated for the previous queries. Having the query which took most execution time along with it's arguments could help users in further investigating performance issues.
When query normalization is disabled utility queries like SELECT 10+20 are now stored as is, instead of SELECT $1+$2. Also when functions or sub queries are created the arguments used internally by the function or subqueries will be replaced by NULL instead of $1..$N. The actual arguments will be displayed when the function or subquery is actually invoked.
Ensures that the denormalization of prepared statements is working, also ensure that a query which takes more time to execute replaces the previous denormalized query.
With the query dernomalization feature, having integer literals used in sql like 1, or 2 could create some confusion on whether those are placeholders or constant values, thus this commit updates the pgsm_query_id regression test to use different integer literals to avoid confusion.
Add a new test case: 1. Execute a prepared statement with larger execution time first. 2. Execute the same prepared statement with cheap execution time. 3. Ensures that the denormalized heavy query is not replaced by the cheaper.
On PG 12, 13, the internal return instruction in the following function:
```
CREATE OR REPLACE FUNCTION add(int, int) RETURNS INTEGER AS
$$
BEGIN
return (select $1 + $2);
END; $$ language plpgsql;
```
Is stored as SELECT (select expr1 + expr2)
On PG 14 onward it's stored just as SELECT (expr1 + expr2)
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PG-156
Before this PR,
pg_stat_monitorwas unable to replace query placeholders when tracking prepared statements ifpg_stat_monitor.pgsm_normalized_queryisoff.With this update
pg_stat_monitornow correctly store query argument values for prepared statements.