Re: Create TOAST table only if AM needs

Hi,

> +
> +/*
> + * Check to see whether the table needs a TOAST table. It does only if
> + * (1) there are any toastable attributes, and (2) the maximum length
> + * of a tuple could exceed TOAST_TUPLE_THRESHOLD. (We don't want to
> + * create a toast table for something like "f1 varchar(20)".)
> + */
> +static bool
> +heapam_needs_toast_table(Relation rel)
> +{
> + int32 data_length = 0;
> + bool maxlength_unknown = false;
> + bool has_toastable_attrs = false;
> + TupleDesc tupdesc = rel->rd_att;
> + int32 tuple_length;
> + int i;
> +
> + for (i = 0; i < tupdesc->natts; i++)
> + {
> + Form_pg_attribute att = TupleDescAttr(tupdesc, i);
> +
> + if (att->attisdropped)
> + continue;
> + data_length = att_align_nominal(data_length, att->attalign);
> + if (att->attlen > 0)
> + {
> + /* Fixed-length types are never toastable */
> + data_length += att->attlen;
> + }
> + else
> + {
> + int32 maxlen = type_maximum_size(att->atttypid,
> + att->atttypmod);
> +
> + if (maxlen < 0)
> + maxlength_unknown = true;
> + else
> + data_length += maxlen;
> + if (att->attstorage != 'p')
> + has_toastable_attrs = true;
> + }
> + }
> + if (!has_toastable_attrs)
> + return false; /* nothing to toast? */
> + if (maxlength_unknown)
> + return true; /* any unlimited-length attrs? */
> + tuple_length = MAXALIGN(SizeofHeapTupleHeader +
> + BITMAPLEN(tupdesc->natts)) +
> + MAXALIGN(data_length);
> + return (tuple_length > TOAST_TUPLE_THRESHOLD);
> +}

I'm ok with adding something roughly like this.

> /* ------------------------------------------------------------------------
> * Planner related callbacks for the heap AM
> @@ -2558,6 +2615,8 @@ static const TableAmRoutine heapam_methods = {
>
> .relation_estimate_size = heapam_estimate_rel_size,
>
> + .needs_toast_table = heapam_needs_toast_table,
> +

I'd rather see this have a relation_ prefix.

Greetings,

Andres Freund