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ce8d5fe0e2
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 <tgl@sss.pgh.pa.us> Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com> Discussion: https://postgr.es/m/CAFj8pRDieSQOPDHD_svvR75875uRejS9cN87FoAC3iXMXS1saQ@mail.gmail.com