PostgreSQL在何处处理 sql查询之二十三

/*
 * estimate_rel_size - estimate # pages and # tuples in a table or index
 *
 * We also estimate the fraction of the pages that are marked all-visible in
 * the visibility map, for use in estimation of index-only scans.
 *
 * If attr_widths isn't NULL, it points to the zero-index entry of the
 * relation's attr_widths[] cache; we fill this in if we have need to compute
 * the attribute widths for estimation purposes.
 */
void
estimate_rel_size(Relation rel, int32 *attr_widths,
                  BlockNumber *pages, double *tuples, double *allvisfrac)
{
    BlockNumber curpages;
    BlockNumber relpages;
    double        reltuples;
    BlockNumber relallvisible;
    double        density;

    switch (rel->rd_rel->relkind)
    {
        case RELKIND_RELATION:
        case RELKIND_INDEX:
        case RELKIND_TOASTVALUE:

            fprintf(stderr,"In %s...(double) rel->rd_rel->reltuples is: 

%lf by process %d\n\n",__FUNCTION__,
                        (double) rel->rd_rel->reltuples, getpid());

            /* it has storage, ok to call the smgr */
            curpages = RelationGetNumberOfBlocks(rel);

            //fprintf(stderr,"In %s...(double) rel->rd_rel->reltuples is: 

%lf by process %d\n\n",__FUNCTION__,

(double) rel->rd_rel->reltuples, getpid());

            /*
             * HACK: if the relation has never yet been vacuumed, use a
             * minimum size estimate of 10 pages.  The idea here is to avoid
             * assuming a newly-created table is really small, even if it
             * currently is, because that may not be true once some data gets
             * loaded into it.    Once a vacuum or analyze cycle has been done
             * on it, it's more reasonable to believe the size is somewhat
             * stable.
             *
             * (Note that this is only an issue if the plan gets cached and
             * used again after the table has been filled.    What we're trying
             * to avoid is using a nestloop-type plan on a table that has
             * grown substantially since the plan was made.  Normally,
             * autovacuum/autoanalyze will occur once enough inserts have
             * happened and cause cached-plan invalidation; but that doesn't
             * happen instantaneously, and it won't happen at all for cases
             * such as temporary tables.)
             *
             * We approximate "never vacuumed" by "has relpages = 0", which
             * means this will also fire on genuinely empty relations.    Not
             * great, but fortunately that's a seldom-seen case in the real
             * world, and it shouldn't degrade the quality of the plan too
             * much anyway to err in this direction.
             *
             * There are two exceptions wherein we don't apply this heuristic.
             * One is if the table has inheritance children.  Totally empty
             * parent tables are quite common, so we should be willing to
             * believe that they are empty.  Also, we don't apply the 10-page
             * minimum to indexes.
             */
            if (curpages < 10 &&
                rel->rd_rel->relpages == 0 &&
                !rel->rd_rel->relhassubclass &&
                rel->rd_rel->relkind != RELKIND_INDEX)
                curpages = 10;

            /* report estimated # pages */
            *pages = curpages;
            /* quick exit if rel is clearly empty */
            if (curpages == 0)
            {
                *tuples = 0;
                *allvisfrac = 0;
                break;
            }

            /* coerce values in pg_class to more desirable types */
            relpages = (BlockNumber) rel->rd_rel->relpages;

            reltuples = (double) rel->rd_rel->reltuples;

            //fprintf(stderr,"In %s...reltuples is: %lf\n by process %d",

__FUNCTION__,reltuples, getpid());

            relallvisible = (BlockNumber) rel->rd_rel->relallvisible;

            /*
             * If it's an index, discount the metapage while estimating the
             * number of tuples.  This is a kluge because it assumes more than
             * it ought to about index structure.  Currently it's OK for
             * btree, hash, and GIN indexes but suspect for GiST indexes.
             */
            if (rel->rd_rel->relkind == RELKIND_INDEX &&
                relpages > 0)
            {
                curpages--;
                relpages--;
            }

            /* estimate number of tuples from previous tuple density */
            if (relpages > 0)
                density = reltuples / (double) relpages;
            else
            {
                /*
                 * When we have no data because the relation was truncated,
                 * estimate tuple width from attribute datatypes.  We assume
                 * here that the pages are completely full, which is OK for
                 * tables (since they've presumably not been VACUUMed yet) but
                 * is probably an overestimate for indexes.  Fortunately
                 * get_relation_info() can clamp the overestimate to the
                 * parent table's size.
                 *
                 * Note: this code intentionally disregards alignment
                 * considerations, because (a) that would be gilding the lily
                 * considering how crude the estimate is, and (b) it creates
                 * platform dependencies in the default plans which are kind
                 * of a headache for regression testing.
                 */
                int32        tuple_width;

                tuple_width = get_rel_data_width(rel, attr_widths);
                tuple_width += sizeof(HeapTupleHeaderData);
                tuple_width += sizeof(ItemPointerData);
                /* note: integer division is intentional here */
                density = (BLCKSZ - SizeOfPageHeaderData) / tuple_width;
            }

            *tuples = rint(density * (double) curpages);

            //fprintf(stderr,"In %s...*tuples is: %lf\n by process %d",

__FUNCTION__,*tuples, getpid());

            /*
             * We use relallvisible as-is, rather than scaling it up like we
             * do for the pages and tuples counts, on the theory that any
             * pages added since the last VACUUM are most likely not marked
             * all-visible.  But costsize.c wants it converted to a fraction.
             */
            if (relallvisible == 0 || curpages <= 0)
                *allvisfrac = 0;
            else if ((double) relallvisible >= curpages)
                *allvisfrac = 1;
            else
                *allvisfrac = (double) relallvisible / curpages;
            break;
        case RELKIND_SEQUENCE:
            /* Sequences always have a known size */
            *pages = 1;
            *tuples = 1;
            *allvisfrac = 0;
            break;
        case RELKIND_FOREIGN_TABLE:
            /* Just use whatever's in pg_class */
            *pages = rel->rd_rel->relpages;
            *tuples = rel->rd_rel->reltuples;
            *allvisfrac = 0;
            break;
        default:
            /* else it has no disk storage; probably shouldn't get here? */
            *pages = 0;
            *tuples = 0;
            *allvisfrac = 0;
            break;
    }
}