Re: BUG #18205: Performance regression with NOT NULL checks.

Hi,

On 2023-11-19 14:41:47 -0800, Andres Freund wrote:
> It doesn't make it algorithmically better, you're right - but I think it's
> quite noticeable even in the case of the other columns having values.
>
> I changed the test to insert 0 insto all columns other than y, and changed the
> WHERE clause to IS NOT NULL, to avoid the overhead of the aggregation
> path. Profile:
>
> -   91.27%     0.00%  postgres  postgres                 [.] ExecProcNode (inlined)
>      ExecProcNode (inlined)
>    - ExecScan
>       - 60.21% ExecQual (inlined)
>          - ExecEvalExprSwitchContext (inlined)
>             - 59.70% ExecInterpExpr
>                - 54.05% slot_getsomeattrs (inlined)
>                   - 53.52% slot_getsomeattrs_int
>                      - 52.93% tts_buffer_heap_getsomeattrs
>                         - 52.88% slot_deform_heap_tuple (inlined)
>                            + 12.39% fetch_att (inlined)
>                            + 12.26% att_isnull (inlined)
>                            + 0.14% asm_sysvec_apic_timer_interrupt
>                + 1.60% BoolGetDatum (inlined)
>                + 0.01% asm_sysvec_apic_timer_interrupt
>               0.35% MemoryContextSwitchTo (inlined)
>       - 30.47% ExecScanFetch (inlined)
>          + 29.87% SeqNext
>          + 0.01% asm_sysvec_apic_timer_interrupt
>         0.23% MemoryContextReset
>       + 0.01% asm_sysvec_apic_timer_interrupt
>
>
> So even here we spend a decent amount of the time in null bitmap handling.

If I put prewarm the data into shared buffers and change the table so there
is a NULL in one of the leading columns, this changes to:

-   94.15%     0.00%  postgres  postgres                 [.] ExecProcNode (inlined)
     ExecProcNode (inlined)
   - ExecScan
      - 79.15% ExecQual (inlined)
         - ExecEvalExprSwitchContext (inlined)
            - 78.50% ExecInterpExpr
               - 71.62% slot_getsomeattrs (inlined)
                  - 70.52% slot_getsomeattrs_int
                     - 69.79% tts_buffer_heap_getsomeattrs
                        - 69.56% slot_deform_heap_tuple (inlined)
                           + 20.25% att_isnull (inlined)
                           + 9.45% fetch_att (inlined)
                           + 0.39% asm_sysvec_apic_timer_interrupt
               + 1.88% BoolGetDatum (inlined)
               + 0.09% asm_sysvec_apic_timer_interrupt
              0.51% MemoryContextSwitchTo (inlined)
            + 0.02% asm_sysvec_apic_timer_interrupt
      + 14.39% ExecScanFetch (inlined)
        0.14% MemoryContextReset
      + 0.03% asm_sysvec_apic_timer_interrupt

Reducing the time spent in att_isnull() wouldn't get us to < 10 timings, but
it'd certainly help to close the gap. Of course you can make the difference
more extreme by adding a lot more leading columns, but still.

Greetings,

Andres Freund