plpgsql: optimize "SELECT simple-expression INTO var". Previously, we always fed SELECT ... INTO to the SPI machinery. While that works for all cases, it's a great deal slower than the otherwise-equivalent "var := expression" if the expression is "simple" and the INTO target is a single variable. Users coming from MSSQL or T_SQL are likely to be surprised by this; they are used to writing SELECT ... INTO since there is no "var := expression" syntax in those dialects. Hence, check for a simple expression and use the faster code path if possible.
(Here, "simple" means whatever exec_is_simple_query accepts, which basically means "SELECT scalar-expression" without any input tables, aggregates, qual clauses, etc.) This optimization is not entirely transparent. Notably, one of the reasons it's faster is that the hooks that pg_stat_statements uses aren't called in this path, so that the evaluated expression no longer appears in pg_stat_statements output as it did before. There may be some other minor behavioral changes too, although I tried hard to make error reporting look the same. Hopefully, none of them are significant enough to not be acceptable as routine changes in a PG major version. Author: Tom Lane <[email protected]> Reviewed-by: Pavel Stehule <[email protected]> Discussion: https://postgr.es/m/cafj8prdiesqopdhd_svvr75875urejs9cn87foac3ixmxs1...@mail.gmail.com Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/ce8d5fe0e2802158b65699aeae1551d489948167 Modified Files -------------- .../pg_stat_statements/expected/level_tracking.out | 8 +- contrib/pg_stat_statements/expected/plancache.out | 6 +- src/pl/plpgsql/src/expected/plpgsql_simple.out | 19 ++++ src/pl/plpgsql/src/pl_exec.c | 100 +++++++++++++++++++++ src/pl/plpgsql/src/sql/plpgsql_simple.sql | 17 ++++ 5 files changed, 141 insertions(+), 9 deletions(-)
