From db989184cda7f4aa1ff764cca96142029e7e093b Mon Sep 17 00:00:00 2001 From: Alvaro Herrera Date: Fri, 28 Feb 2020 13:13:54 -0300 Subject: [PATCH] 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 --- src/backend/tcop/postgres.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c index 0a6f80963b..23661ae15f 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();