Re: Introduce new multi insert Table AM and improve performance of various SQL commands with it for Heap AM

On Mon, Apr 29, 2024 at 11:36 AM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> Please see the attached v20 patch set.

It looks like with the use of the new multi insert table access method
(TAM) for COPY (v20-0005), pgbench regressed about 35% [1]. The reason
is that the memory-based flushing decision the new TAM takes [2]
differs from that of what the COPY does today with table_multi_insert.
The COPY with table_multi_insert, maintains exact size of the tuples
in CopyFromState after it does the line parsing. For instance, the
tuple size of a table with two integer columns is 8 (4+4) bytes here.
The new TAM relies on the memory occupied by the slot's memory context
which holds the actual tuple as a good approximation for the tuple
size. But, this memory context size also includes a tuple header, so
the size here is not just 8 (4+4) bytes but more. Because of this, the
buffers get flushed sooner than that of the existing COPY with
table_multi_insert AM causing regression in pgbench which uses COPY
extensively. The new TAM aren't designed to be able to receive tuple
sizes from the callers, even if we do that, the API doesn't look
generic.

Here are couple of ideas to get away with this:

1. Try to get the actual tuple sizes excluding header sizes for each
column in the new TAM.
2. Try not to use the new TAM for COPY in which case the
table_multi_insert stays forever.
3. Try passing a flag to tell the new TAM that the caller does the
flushing and no need for an internal flushing.

I haven't explored the idea (1) in depth yet. If we find a way to do
so, it looks to me that we are going backwards since we need to strip
off headers for each column of a row for all of the rows. I suspect
this would cost a bit more and may not solve the regression.

With an eventual goal to get rid of table_multi_insert, (3) may not be
a better choice.

(3) seems reasonable to implement and reduce the regression. I did so
in the attached v21 patches. A new flag TM_SKIP_INTERNAL_BUFFER_FLUSH
is introduced in v21 patch, when specified, no internal flushing is
done, the caller has to flush the buffered tuples using
table_modify_buffer_flush(). Check the test results [3] HEAD 2.948 s,
PATCHED 2.946 s.

v21 also adds code to maintain tuple size for virtual tuple slots.
This helps make better memory-based flushing decisions in the new TAM.

Thoughts?

[1]
HEAD:
done in 2.84 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.99 s, vacuum 0.21 s, primary keys 0.62 s).
done in 2.78 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.88 s, vacuum 0.21 s, primary keys 0.69 s).
done in 2.97 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.07 s, vacuum 0.21 s, primary keys 0.69 s).
done in 2.86 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.96 s, vacuum 0.21 s, primary keys 0.69 s).
done in 2.90 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.05 s, vacuum 0.21 s, primary keys 0.64 s).
done in 2.83 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.96 s, vacuum 0.21 s, primary keys 0.66 s).
done in 2.80 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.95 s, vacuum 0.20 s, primary keys 0.63 s).
done in 2.79 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 1.89 s, vacuum 0.21 s, primary keys 0.69 s).
done in 3.75 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.17 s, vacuum 0.32 s, primary keys 1.25 s).
done in 3.86 s (drop tables 0.00 s, create tables 0.08 s, client-side
generate 2.97 s, vacuum 0.21 s, primary keys 0.59 s).

AVG done in 2.948 s

v20 PATCHED:
done in 3.94 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.12 s, vacuum 0.19 s, primary keys 0.62 s).
done in 4.04 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.22 s, vacuum 0.20 s, primary keys 0.61 s).
done in 3.98 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.16 s, vacuum 0.20 s, primary keys 0.61 s).
done in 4.04 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.16 s, vacuum 0.20 s, primary keys 0.67 s).
done in 3.98 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.16 s, vacuum 0.20 s, primary keys 0.61 s).
done in 4.00 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.17 s, vacuum 0.20 s, primary keys 0.63 s).
done in 4.43 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.24 s, vacuum 0.21 s, primary keys 0.98 s).
done in 4.16 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 3.36 s, vacuum 0.20 s, primary keys 0.59 s).
done in 3.62 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.83 s, vacuum 0.20 s, primary keys 0.58 s).
done in 3.67 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.84 s, vacuum 0.21 s, primary keys 0.61 s).

AVG done in 3.986 s

[2]
+ /*
+ * Memory allocated for the whole tuple is in slot's memory context, so
+ * use it keep track of the total space occupied by all buffered tuples.
+ */
+ if (TTS_SHOULDFREE(slot))
+ mistate->cur_size += MemoryContextMemAllocated(slot->tts_mcxt, false);
+
+ if (mistate->cur_slots >= HEAP_MAX_BUFFERED_SLOTS ||
+ mistate->cur_size >= HEAP_MAX_BUFFERED_BYTES)
+ heap_modify_buffer_flush(state);

[3]
v21 PATCHED:
done in 2.92 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.12 s, vacuum 0.21 s, primary keys 0.59 s).
done in 2.89 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.07 s, vacuum 0.21 s, primary keys 0.61 s).
done in 2.89 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.05 s, vacuum 0.21 s, primary keys 0.62 s).
done in 2.90 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.07 s, vacuum 0.21 s, primary keys 0.62 s).
done in 2.80 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.00 s, vacuum 0.21 s, primary keys 0.59 s).
done in 2.84 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.04 s, vacuum 0.20 s, primary keys 0.60 s).
done in 2.84 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.03 s, vacuum 0.20 s, primary keys 0.59 s).
done in 2.85 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.04 s, vacuum 0.20 s, primary keys 0.60 s).
done in 3.48 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.44 s, vacuum 0.23 s, primary keys 0.80 s).
done in 3.05 s (drop tables 0.00 s, create tables 0.01 s, client-side
generate 2.28 s, vacuum 0.21 s, primary keys 0.55 s).

AVG done in 2.946 s

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com