Subject: Re: PoC/WIP: Extended statistics on expressions
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Dean Rasheed <dean.a.rasheed@gmail.com>, pgsql-hackers@postgresql.org
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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <131b0d63-5ab8-6fa4-fb77-cd0bc23d5e23@enterprisedb.com>
Date: Mon, 18 Jan 2021 00:26:17 +0100
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On 1/17/21 6:29 AM, Justin Pryzby wrote:
> On Sun, Jan 17, 2021 at 01:23:39AM +0100, Tomas Vondra wrote:
>> diff --git a/doc/src/sgml/ref/create_statistics.sgml b/doc/src/sgml/ref/create_statistics.sgml
>> index 4363be50c3..5b8eb8d248 100644
>> --- a/doc/src/sgml/ref/create_statistics.sgml
>> +++ b/doc/src/sgml/ref/create_statistics.sgml
>> @@ -21,9 +21,13 @@ PostgreSQL documentation
>>   
>>    <refsynopsisdiv>
>>   <synopsis>
>> +CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_name</replaceable>
>> +    ON ( <replaceable class="parameter">expression</replaceable> )
>> +    FROM <replaceable class="parameter">table_name</replaceable>
> 
>>   CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_name</replaceable>
>>       [ ( <replaceable class="parameter">statistics_kind</replaceable> [, ... ] ) ]
>> -    ON <replaceable class="parameter">column_name</replaceable>, <replaceable class="parameter">column_name</replaceable> [, ...]
>> +    ON { <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [, ...]
>>       FROM <replaceable class="parameter">table_name</replaceable>
>>   </synopsis>
> 
> I think this part is wrong, since it's not possible to specify a single column
> in form#2.
> 
> If I'm right, the current way is:
> 
>   - form#1 allows expression statistics of a single expression, and doesn't
>     allow specifying "kinds";
> 
>   - form#2 allows specifying "kinds", but always computes expression statistics,
>     and requires multiple columns/expressions.
> 
> So it'd need to be column_name|expression, column_name|expression [,...]
> 

Strictly speaking you're probably correct - there should be at least two 
elements. But I'm somewhat hesitant about making this more complex, 
because it'll be harder to understand.


>> @@ -39,6 +43,16 @@ CREATE STATISTICS [ IF NOT EXISTS ] <replaceable class="parameter">statistics_na
>>      database and will be owned by the user issuing the command.
>>     </para>
>>   
>> +  <para>
>> +   The <command>CREATE STATISTICS</command> command has two basic forms. The
>> +   simple variant allows building statistics for a single expression, does
>> +   not allow specifying any statistics kinds and provides benefits similar
>> +   to an expression index. The full variant allows defining statistics objects
>> +   on multiple columns and expressions, and selecting which statistics kinds will
>> +   be built. The per-expression statistics are built automatically when there
>> +   is at least one expression.
>> +  </para>
> 
>> +   <varlistentry>
>> +    <term><replaceable class="parameter">expression</replaceable></term>
>> +    <listitem>
>> +     <para>
>> +      The expression to be covered by the computed statistics. In this case
>> +      only a single expression is required, in which case only the expression
>> +      statistics kind is allowed. The order of expressions is insignificant.
> 
> I think this part is wrong now ?
> I guess there's no user-facing expression "kind" left in the CREATE command.
> I guess "in which case" means "if only one expr is specified".
> "expression" could be either form#1 or form#2.
> 
> Maybe it should just say:
>> +      An expression to be covered by the computed statistics.
> 
> Maybe somewhere else, say:
>> In the second form of the command, the order of expressions is insignificant.
> 

Yeah, this is a leftover from when there was "expressions" kind. I'll 
reword this a bit.


thanks

-- 
Tomas Vondra
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