Horton

Re: Remove redundant MemoryContextSwith in BeginCopyFrom

Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>

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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
To: Japin Li <japinli@hotmail.com>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2022-01-19T15:35:09Z
Lists: pgsql-hackers

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  1. Remove redundant memory context switches in BeginCopyFrom().

On Wed, Jan 19, 2022 at 7:51 PM Japin Li <japinli@hotmail.com> wrote:
>
>
> Hi, hackers
>
> When I read the code of COPY ... FROM ..., I find there is a redundant
> MemoryContextSwith() in BeginCopyFrom().  In BeginCopyFrom, it creates
> a COPY memory context and then switches to it, in the middle of this
> function, it switches to the oldcontext and immediately switches back to
> COPY memory context, IMO, this is redundant, and can be removed safely.

+1. It looks like a thinko from c532d15d. There's no code in between,
so switching to oldcontext doesn't make sense.

    MemoryContextSwitchTo(oldcontext);
    << no code here >>
    oldcontext = MemoryContextSwitchTo(cstate->copycontext);

I think we also need to remove MemoryContextSwitchTo(oldcontext); at
the end of BeginCopyTo in copyto.c, because we are not changing memory
contexts in between.

diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 34c8b80593..5182048e4f 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -742,8 +742,6 @@ BeginCopyTo(ParseState *pstate,

        cstate->bytes_processed = 0;

-       MemoryContextSwitchTo(oldcontext);
-
        return cstate;
 }

Regards,
Bharath Rupireddy.