Re: unnecessary executor overheads around seqscans

On Sat, 24 Jan 2026 at 19:21, Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> On Sat, Jan 24, 2026 at 5:16 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > We can use a pg_assume() in table_scan_getnextslot() to make the compiler
> > understand.
>
> Something like this?
>
> result = sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, slot);
> pg_assume(result == !TupIsNull(slot));
> return result;
>
> I assume this relies on table_scan_getnextslot() being inlined into
> ExecScanExtended()?

I looked at the objdump of this, and it does seem to get rid of the extra check.

I did:

cd src/backend/executor
gcc -Wall -Wmissing-prototypes -Wpointer-arith
-Wdeclaration-after-statement -Werror=vla -Wendif-labels
-Wmissing-format-attribute -Wimplicit-fallthrough=3
-Wcast-function-type -Wshadow=compatible-local -Wformat-security
-Wmissing-variable-declarations -fno-strict-aliasing -fwrapv
-fexcess-precision=standard -Wno-format-truncation
-Wno-stringop-truncation -O2 -I../../../src/include -D_GNU_SOURCE -g
-c nodeSeqscan.c
objdump -d -M intel -S nodeSeqscan.o > nodeSeqscan_pg_assume.s

Looking at ExecSeqScanWithQual, I see:

master:
return sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, slot);
22c: 48 8b 07 mov rax,QWORD PTR [rdi]
22f: 4c 89 f2 mov rdx,r14
232: 44 89 fe mov esi,r15d
235: 48 8b 80 40 01 00 00 mov rax,QWORD PTR [rax+0x140]
23c: ff 50 28 call QWORD PTR [rax+0x28]
if (table_scan_getnextslot(scandesc, direction, slot))
23f: 84 c0 test al,al <-- *** this test and the
subsequent jump equal are removed
241: 74 6d je 2b0 <ExecSeqScanWithQual+0x100>
if (TupIsNull(slot))
243: 41 f6 46 04 02 test BYTE PTR [r14+0x4],0x2
248: 75 69 jne 2b3 <ExecSeqScanWithQual+0x103>
24a: 48 8b 45 28 mov rax,QWORD PTR [rbp+0x28]

And with your pg_assume added:
result = sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction, slot);
22c: 48 8b 07 mov rax,QWORD PTR [rdi]
22f: 4c 89 f2 mov rdx,r14
232: 44 89 fe mov esi,r15d
235: 48 8b 80 40 01 00 00 mov rax,QWORD PTR [rax+0x140]
23c: ff 50 28 call QWORD PTR [rax+0x28]
pg_assume(result == !TupIsNull(slot));
23f: 41 f6 46 04 02 test BYTE PTR [r14+0x4],0x2
244: 75 62 jne 2a8 <ExecSeqScanWithQual+0xf8>
246: 48 8b 45 28 mov rax,QWORD PTR [rbp+0x28]

I didn't test the performance.

David