Previously the stats.sql regression test used conditions like
"datname = (SELECT current_database())" to check the current database name.
The subquery is unnecessary, so this commit simplifies these expressions to
"datname = current_database()".
Author: Chao Li <lic@highgo.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/A1535A8F-65AF-4C3D-ACBE-25891CB5D38B@gmail.com
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