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PG-223: Remove relations and num_relations from pgssSharedState.

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These variables can't be in shared state, as the following problem was
taking place:
1. Process1 call pgss_ExecutorCheckPerms(), acquire lock, update
   relations and num_relations, release lock.
2. Process 2 call pgss_ExecutorCheckPerms(), acquire lock, update
   num_relations = 0;
3. Process 1 read num_relations = 0 in pgss_update_entry, this value is
   wrong as it was updated by Process 2.

Even if we acquire the lock in pgss_update_entry to read num_relations
and relations variable, Process 1 may end up acquiring the lock after
Process 2 has ovewritten the variable values, leading to Process 1
reading of wrong data.

By defining relations and num_relations to be static and global in
pg_stat_monitor.c we take advantage that each individual PostgreSQL
backend will have its own copy of this data, which allows us to remove
the locking in pgss_ExecutorCheckPerms to update these variables,
improving pg_stat_monitor overall performance.
This commit is contained in:
Diego Fronza
2021-08-27 15:53:11 -04:00
parent f3962509fb
commit a847ed95de
2 changed files with 9 additions and 15 deletions
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2
pg_stat_monitor.h
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@@ -310,8 +310,6 @@ typedef struct pgssSharedState
uint64 prev_bucket_usec;
uint64 bucket_entry[MAX_BUCKETS];
int64 query_buf_size_bucket;
char relations[REL_LST][REL_LEN];
int num_relations; /* Number of relation in the query */
char bucket_start_time[MAX_BUCKETS][60]; /* start time of the bucket */
} pgssSharedState;