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Date: Tue, 5 May 2026 14:48:30 +0700
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Subject: Re: Pgbench: remove synchronous prepare
To: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org
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From: Dmitrii Bondar <d.bondar@postgrespro.ru>
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On 4/30/26 4:46 AM, Robert Haas wrote:
> Why isn't the solution to use the existing PQsendPrepare function
> instead of adding a new libpq entrypoint?

pgbench is not designed to process a response to a Parse message alone, 
because of meta-commands. For example, the \gset command requires a 
tuple to be stored, but a response to a Parse message does not provide 
one. This leads to the error: pgbench: error: client 0 script 0 command 
0 query 0: expected one row, got 0. Sending all additional messages with 
PQsendQueryPrepared may look like the exact solution, but it is not. 
PQsendQueryStart does not allow more than one command to be sent unless 
pipeline mode is enabled. This could be fixed in two ways: either by 
allowing libpq to send more than one command when pipeline is disabled, 
or by adding a new state-machine state to pgbench. Both options seem 
more invasive than the current solution. Adding a new libpq function 
just for pgbench (at least for now) does not seem ideal either, but it 
may be simpler and safer.

> I don't think we
> can add a function with a name like PQsendPBES, and I think we need to

I have other suggestions:
"PQsendQueryPrepare" but it is too close to the existing name 
"PQsendQueryPrepared".
"PQsendPrepareQuery" similar to "PQsendPrepare" but it also executes the 
query.
"PQsendPrepareExecute" is not especially well aligned with the existing 
naming scheme, but it may describe the intent quite well.


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    <p><br>
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    <div class="moz-cite-prefix">On 4/30/26 4:46 AM, Robert Haas wrote:<span
      style="white-space: pre-wrap">
</span></div>
    <blockquote type="cite"
cite="mid:CA+TgmoYnWeA_jOOdebo=vfaCFAUYk2mMd5cPm4a_FnSMu_4ozA@mail.gmail.com">
      <pre wrap="" class="moz-quote-pre">Why isn't the solution to use the existing PQsendPrepare function
instead of adding a new libpq entrypoint?
</pre>
    </blockquote>
    <p>pgbench is not designed to process a response to a Parse message
      alone, because of meta-commands. For example, the \gset command
      requires a tuple to be stored, but a response to a Parse message
      does not provide one. This leads to the error: pgbench: error:
      client 0 script 0 command 0 query 0: expected one row, got 0.
      Sending all additional messages with PQsendQueryPrepared may look
      like the exact solution, but it is not. PQsendQueryStart does not
      allow more than one command to be sent unless pipeline mode is
      enabled. This could be fixed in two ways: either by allowing libpq
      to send more than one command when pipeline is disabled, or by
      adding a new state-machine state to pgbench. Both options seem
      more invasive than the current solution. Adding a new libpq
      function just for pgbench (at least for now) does not seem ideal
      either, but it may be simpler and safer.</p>
    <pre wrap="" class="moz-quote-pre"><blockquote type="cite"><pre
    wrap="" class="moz-quote-pre">I don't think we
can add a function with a name like PQsendPBES, and I think we need to</pre></blockquote></pre>
    <p>I have other suggestions:<br>
      "PQsendQueryPrepare" but it is too close to the existing name
      "PQsendQueryPrepared".<br>
      "PQsendPrepareQuery" similar to "PQsendPrepare" but it also
      executes the query. <br>
      "PQsendPrepareExecute" is not especially well aligned with the
      existing naming scheme, but it may describe the intent quite well.</p>
    <pre wrap="" class="moz-quote-pre"></pre>
    <p><br>
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