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From: Daniele Varrazzo <daniele.varrazzo@gmail.com>
Date: Thu, 24 Dec 2020 00:14:23 +0000
Message-ID: 
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Subject: Re: psycopg3, prepared statements
To: Vladimir Ryabtsev <greatvovan@gmail.com>
Cc: Adrian Klaver <adrian.klaver@aklaver.com>, psycopg@postgresql.org
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On Wed, 23 Dec 2020 at 23:23, Vladimir Ryabtsev <greatvovan@gmail.com> wrote:
>
> Cause (query, types) can give more combinations than (query,)?

Yes, that's the reason

    In [1]: import psycopg3
    In [2]: cnn = psycopg3.connect()
    In [3]: cnn.prepare_threshold = 2

    In [4]: cnn.execute("select 1 + %s", [1]).fetchone()
    Out[4]: (2,)
    In [5]: cnn.execute("select 1 + %s", [None]).fetchone()
    Out[5]: (None,)
    In [7]: cnn.execute("select 1 + %s", [2]).fetchone()
    Out[7]: (3,)

After 3 times the expression should have been prepared, but the tally
has been spread in two values (0 is unknown oid, 20 is int oid).

    In [8]: cnn._prepared_statements
    Out[8]: OrderedDict([((b'select 1 + $1', (0,)), 1), ((b'select 1 +
$1', (20,)), 2)])

    In [9]: cnn.execute("select 1 + %s", [3]).fetchone()
    Out[9]: (4,)

However, when either key passes the threshold, eventually preparation happens.

    In [10]: cnn._prepared_statements
    Out[10]:
    OrderedDict([((b'select 1 + $1', (0,)), 1),
                ((b'select 1 + $1', (20,)), b'_pg3_0')])

_pg3_0 is the name under which that combination of query and types is
now prepared (it is local per session).

-- Daniele