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From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Date: Fri, 21 Jun 2024 17:25:51 +0530
Message-ID: <CAExHW5tFq_5F=NLLsmyBrnYLTjmN_q7RgrbMYxwZz8mxbeQgcQ@mail.gmail.com>
Subject: Re: Custom TupleTableSlotOps while Initializing Custom Scan
To: V N G Samba Siva Reddy Chinta <sambasivareddychinta@gmail.com>
Cc: pgsql-hackers@postgresql.org
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Hi,

On Thu, Jun 20, 2024 at 5:58=E2=80=AFPM V N G Samba Siva Reddy Chinta <
sambasivareddychinta@gmail.com> wrote:

> Hello Team,
> Good Day,
>
> I have been working on adding a CustomScanState object in the executor
> state in my project. As part of CustomScanState, I execute queries and
> store their results in the Tuplestorestate object. After storing all
> tuples in the Tuplestorestate, I retrieve each tuple and place it in the
> TupleTableSlot using the tuplestore_gettupleslot() function.
>
> However, I encounter an error: *"trying to store a minimal tuple into the
> wrong type of slot."* Upon debugging, I discovered that the TupleTableSlo=
t
> only holds virtual tuples (tupleTableSlot->tts_ops is set to TTSOpsVirtua=
l).
> In contrast, tuplestore_gettupleslot() calls ExecStoreMinimalTuple(),
> which expects TupleTableSlotOps of type TTSOpsMinimalTuple.
>
> Further investigation revealed that in the ExecInitCustomScan() function
> within the nodeCustom.c source file, where ScanTupleSlot and
> ResultTupleSlots are initialized, users can choose custom slots by
> setting slotOps in CustomScanState. We initialize the ScanTupleSlot based
> on user-specified slotOps, but for ResultTupleSlot, we proceed with
> TTSOpsVirtual instead of the custom slotOps, which is causing the issue.
>
> Is this behavior expected? Is there a way to store tuples in slots
> according to the TupleTableSlot type?
>
> I found a function ExecForceStoreMinimalTuple() which can be used in my
> case. We need to pass the MinimalTuple to this function, but I was unable
> to find a way to fetch the tuple from tuple storestate. We do have tuples=
tore_gettuple()
> function to get the minimal tuple but it is a static function, is there a=
ny
> other function like that?
>

From the description I assume that you are passing ResultTupleSlot to
tuplestore_gettupleslot(). Please confirm.

I think the reason for CustomScan's ResultTupleSlot to be set to virtual
tuple slot is it's the most general kind of tuple - values and isnulls.
Other kinds depend upon the heap tuple format. And the rest of the code
needs CustomScans to produce a deterministic tuple slot type.

Without knowing the details of your code, I think what you need to do is
declare a minimal tuple slot, fetch the tuple from store using this slot
and then store it into ResultTupleSlot using its ttsops. Other types of
planstate nodes some times return the slot they get from their subplans as
is without going through ResultTupleSlot. You may want to do something like
that, if the result tuple contents are same as the tuple stored in
tuplestore

--=20
Best Wishes,
Ashutosh Bapat

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<div dir=3D"ltr"><div>Hi,</div><br><div class=3D"gmail_quote"><div dir=3D"l=
tr" class=3D"gmail_attr">On Thu, Jun 20, 2024 at 5:58=E2=80=AFPM V N G Samb=
a Siva Reddy Chinta &lt;<a href=3D"mailto:sambasivareddychinta@gmail.com">s=
ambasivareddychinta@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"=
gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(20=
4,204,204);padding-left:1ex"><div dir=3D"ltr"><div>Hello Team,<br></div><di=
v>Good Day,</div><p>I have been working on adding a <code>CustomScanState</=
code> object in the executor state in my project. As part of <code>CustomSc=
anState</code>, I execute queries and store their results in the <code>Tupl=
estorestate</code> object. After storing all tuples in the <code>Tuplestore=
state</code>, I retrieve each tuple and place it in the <code>TupleTableSlo=
t</code> using the <code>tuplestore_gettupleslot()</code> function.</p><p>H=
owever, I encounter an error: <b>&quot;trying to store a minimal tuple into=
 the wrong type of slot.&quot;</b> Upon debugging, I discovered that the <c=
ode>TupleTableSlot</code> only holds virtual tuples (<code>tupleTableSlot-&=
gt;tts_ops</code> is set to <code>TTSOpsVirtual</code>). In contrast, <code=
>tuplestore_gettupleslot()</code> calls <code>ExecStoreMinimalTuple()</code=
>, which expects <code>TupleTableSlotOps</code> of type <code>TTSOpsMinimal=
Tuple</code>.</p><p>Further investigation revealed that in the <code>ExecIn=
itCustomScan()</code> function within the <code>nodeCustom.c</code> source =
file, where <code>ScanTupleSlot</code> and <code>ResultTupleSlots</code> ar=
e initialized, users can choose custom slots by setting <code>slotOps</code=
> in <code>CustomScanState</code>. We initialize the <code>ScanTupleSlot</c=
ode> based on user-specified <code>slotOps</code>, but for <code>ResultTupl=
eSlot</code>, we proceed with <code>TTSOpsVirtual</code> instead of the cus=
tom <code>slotOps</code>, which is causing the issue<code>.</code></p><p>Is=
 this behavior expected? Is there a way to store tuples in slots according =
to the <code>TupleTableSlot</code> type?</p><p><code>I found a function=C2=
=A0</code><code>ExecForceStoreMinimalTuple()</code><code> which can be used=
 in my case. We need to pass the MinimalTuple to this function, but I was u=
nable to find a way to fetch the tuple from tuple storestate. We do have </=
code><span style=3D"font-family:monospace">tuplestore_gettuple() function t=
o get the minimal tuple but it is a static function, is there any other fun=
ction like that?</span></p></div></blockquote><div><br></div><div>From the =
description I assume that you are passing ResultTupleSlot to tuplestore_get=
tupleslot(). Please confirm.</div><div><br></div><div>I think the reason fo=
r CustomScan&#39;s ResultTupleSlot to be set to virtual tuple slot is it=
9;s the most general kind of tuple - values and isnulls. Other kinds depend=
 upon the heap tuple format. And the rest of the code needs CustomScans to =
produce a deterministic tuple slot type.<br></div><div><br></div><div>Witho=
ut knowing the details of your code, I think what you need to do is declare=
 a minimal tuple slot, fetch the tuple from store using this slot and then =
store it into ResultTupleSlot using its ttsops. Other types of planstate no=
des some times return the slot they get from their subplans as is without g=
oing through ResultTupleSlot. You may want to do something like that, if th=
e result tuple contents are same as the tuple stored in tuplestore</div></d=
iv><div><br></div><span class=3D"gmail_signature_prefix">-- </span><br><div=
 dir=3D"ltr" class=3D"gmail_signature"><div dir=3D"ltr"><div><div dir=3D"lt=
r"><div>Best Wishes,</div><div>Ashutosh Bapat<br></div></div></div></div></=
div></div>

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