Date: Tue, 31 Mar 2020 13:31:26 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Michael Paquier <michael@paquier.xyz>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>, Simon Riggs <simon@2ndquadrant.com>,
	Shaun Thomas <shaun.thomas@2ndquadrant.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>,
	Andreas Karlsson <andreas@proxel.se>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
Message-ID: <20200331163126.GA6532@alvherre.pgsql>
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On 2020-Mar-30, James Coleman wrote:

> +/* ----------------
> + *	 Instruementation information for IncrementalSort
> + * ----------------
> + */
> +typedef struct IncrementalSortGroupInfo
> +{
> +	int64		groupCount;
> +	long		maxDiskSpaceUsed;
> +	long		totalDiskSpaceUsed;
> +	long		maxMemorySpaceUsed;
> +	long		totalMemorySpaceUsed;
> +	Size		sortMethods; /* bitmask of TuplesortMethod */
> +} IncrementalSortGroupInfo;

There's a typo "Instruementation" in the comment, but I'm more surprised
that type Size is being used to store a bitmask.  It looks weird to me.
Wouldn't it be more reasonable to use bits32 or some such?  (I first
noticed this in the "sizeof(Size)" code that appears in the explain
code.)


OTOH, aesthetically it would seem to be better to define these values
using ones and increasing shifts (1 << 1 and so on), rather than powers
of two:

> + * TuplesortMethod is used in a bitmask in Increment Sort's shared memory
> + * instrumentation so needs to have each value be a separate bit.
>   */
>  typedef enum
>  {
>  	SORT_TYPE_STILL_IN_PROGRESS = 0,
> -	SORT_TYPE_TOP_N_HEAPSORT,
> -	SORT_TYPE_QUICKSORT,
> -	SORT_TYPE_EXTERNAL_SORT,
> -	SORT_TYPE_EXTERNAL_MERGE
> +	SORT_TYPE_TOP_N_HEAPSORT = 2,
> +	SORT_TYPE_QUICKSORT = 4,
> +	SORT_TYPE_EXTERNAL_SORT = 8,
> +	SORT_TYPE_EXTERNAL_MERGE = 16
>  } TuplesortMethod;

I don't quite understand why you skipped "1".  (Also, is the use of zero
a wise choice?)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services