diff options
| author | Alvaro Herrera <alvherre@alvh.no-ip.org> | 2020-02-28 13:13:54 -0300 |
|---|---|---|
| committer | Alvaro Herrera <alvherre@alvh.no-ip.org> | 2020-02-28 13:25:26 -0300 |
| commit | db989184cda7f4aa1ff764cca96142029e7e093b (patch) | |
| tree | db5304a6d2fda33a4f0ea55686a7d69cc3a75fcd /src/backend/tcop/postgres.c | |
| parent | 1933ae629e7b706c6c23673a381e778819db307d (diff) | |
| download | postgresql-db989184cda7f4aa1ff764cca96142029e7e093b.tar.gz postgresql-db989184cda7f4aa1ff764cca96142029e7e093b.zip | |
Add comments on avoid reuse of parse-time snapshot
Apparently, reusing the parse-time query snapshot for later steps
(execution) is a frequently considered optimization ... but it doesn't
work, for reasons discovered in thread [1]. Adding some comments about
why it doesn't really work can relieve some future hackers from wasting
time reimplementing it again.
[1] https://postgr.es/m/flat/5075D8DF.6050500@fuzzy.cz
Author: Michail Nikolaev
Discussion: https://postgr.es/m/CANtu0ogp6cTvMJObXP8n=k+JtqxY1iT9UV5MbGCpjjPa5crCiw@mail.gmail.com
Diffstat (limited to 'src/backend/tcop/postgres.c')
| -rw-r--r-- | src/backend/tcop/postgres.c | 11 |
1 files changed, 10 insertions, 1 deletions
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c index 0a6f80963bd..23661ae15f5 100644 --- a/src/backend/tcop/postgres.c +++ b/src/backend/tcop/postgres.c @@ -1155,7 +1155,16 @@ exec_simple_query(const char *query_string) plantree_list = pg_plan_queries(querytree_list, CURSOR_OPT_PARALLEL_OK, NULL); - /* Done with the snapshot used for parsing/planning */ + /* + * Done with the snapshot used for parsing/planning. + * + * While it looks promising to reuse the same snapshot for query + * execution (at least for simple protocol), unfortunately it causes + * execution to use a snapshot that has been acquired before locking + * any of the tables mentioned in the query. This creates user- + * visible anomalies, so refrain. Refer to + * https://postgr.es/m/flat/5075D8DF.6050500@fuzzy.cz for details. + */ if (snapshot_set) PopActiveSnapshot(); |