I keep looking at 0001, and in the struct definition I think putting the escontext at the bottom is not great, because there's a comment a few lines above that says "XXX: following fields only needed during "compilation"), could be thrown away afterwards". This comment is not strictly true, because innermost_caseval is actually used by array_map(); yet it seems that ->escontext should appear before that comment.
However, if you put it before steps_len, it would push members steps_len and steps_alloc beyond the struct's first cache line(*). If those struct members are critical for expression init performance, then maybe it's not a good tradeoff. I don't know if this was struct laid out carefully with that consideration in mind or not. Also, ->escontext's own comment in ExprState seems to be saying too much and not saying enough. I would reword it as "For expression nodes that support soft errors. NULL if caller wants them thrown instead". The shortest I could make so that it fits in a single is "For nodes that can error softly. NULL if caller wants them thrown", or "For soft-error-enabled nodes. NULL if caller wants errors thrown". Not sure if those are good enough, or just make the comment the whole four lines ... (*) This is what pahole says about the struct as 0001 would put it: struct ExprState { NodeTag type; /* 0 4 */ uint8 flags; /* 4 1 */ _Bool resnull; /* 5 1 */ /* XXX 2 bytes hole, try to pack */ Datum resvalue; /* 8 8 */ TupleTableSlot * resultslot; /* 16 8 */ struct ExprEvalStep * steps; /* 24 8 */ ExprStateEvalFunc evalfunc; /* 32 8 */ Expr * expr; /* 40 8 */ void * evalfunc_private; /* 48 8 */ int steps_len; /* 56 4 */ int steps_alloc; /* 60 4 */ /* --- cacheline 1 boundary (64 bytes) --- */ struct PlanState * parent; /* 64 8 */ ParamListInfo ext_params; /* 72 8 */ Datum * innermost_caseval; /* 80 8 */ _Bool * innermost_casenull; /* 88 8 */ Datum * innermost_domainval; /* 96 8 */ _Bool * innermost_domainnull; /* 104 8 */ ErrorSaveContext * escontext; /* 112 8 */ /* size: 120, cachelines: 2, members: 18 */ /* sum members: 118, holes: 1, sum holes: 2 */ /* last cacheline: 56 bytes */ }; -- Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/ "Nunca se desea ardientemente lo que solo se desea por razón" (F. Alexandre)