Re: PG 15 (and to a smaller degree 14) regression due to ExprEvalStep size

Hi,

On 2022-06-16 16:31:30 -0700, Andres Freund wrote:
> The EEOP_JSONEXPR stuff was added during 15 development in:
>
> commit 1a36bc9dba8eae90963a586d37b6457b32b2fed4
> Author: Andrew Dunstan 
> Date:   2022-03-03 13:11:14 -0500
>
>     SQL/JSON query functions

I'm quite confused about part of the struct definition of this:

			struct JsonCoercionsState
			{
				struct JsonCoercionState
				{
					JsonCoercion *coercion; /* coercion expression */
					ExprState  *estate; /* coercion expression state */
				}			null,
							string,
				numeric    ,
							boolean,
							date,
							time,
							timetz,
							timestamp,
							timestamptz,
							composite;
			}			coercions;	/* states for coercion from SQL/JSON item
									 * types directly to the output type */

Why on earth do we have coercion state for all these different types? That
really adds up:

                struct {
                        JsonExpr * jsexpr;               /*    24     8 */
                        struct {
                                FmgrInfo func;           /*    32    48 */
                                /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
                                Oid typioparam;          /*    80     4 */
                        } input;                         /*    32    56 */

                        /* XXX last struct has 4 bytes of padding */

                        NullableDatum * formatted_expr;  /*    88     8 */
                        NullableDatum * res_expr;        /*    96     8 */
                        NullableDatum * coercion_expr;   /*   104     8 */
                        NullableDatum * pathspec;        /*   112     8 */
                        ExprState * result_expr;         /*   120     8 */
                        /* --- cacheline 2 boundary (128 bytes) --- */
                        ExprState * default_on_empty;    /*   128     8 */
                        ExprState * default_on_error;    /*   136     8 */
                        List *     args;                 /*   144     8 */
                        void *     cache;                /*   152     8 */
                        struct JsonCoercionsState coercions; /*   160   160 */
                } jsonexpr;                              /*    24   296 */

And why is FmgrInfo stored inline in the struct? Everything else just stores
pointers to FmgrInfo.


Now that I look at this: It's a *bad* idea to have subsidiary ExprState inside
an ExprState. Nearly always the correct thing to do is to build those
expressions. There's plenty memory and evaluation overhead in jumping to a
different expression. And I see no reason for doing it that way here?

This stuff doesn't look ready.

Greetings,

Andres Freund