file_fdw.c

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/*-------------------------------------------------------------------------
 *
 * file_fdw.c
 *        foreign-data wrapper for server-side flat files.
 *
 * Copyright (c) 2010-2013, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        contrib/file_fdw/file_fdw.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <sys/stat.h>
#include <unistd.h>

#include "access/htup_details.h"
#include "access/reloptions.h"
#include "access/sysattr.h"
#include "catalog/pg_foreign_table.h"
#include "commands/copy.h"
#include "commands/defrem.h"
#include "commands/explain.h"
#include "commands/vacuum.h"
#include "foreign/fdwapi.h"
#include "foreign/foreign.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "optimizer/cost.h"
#include "optimizer/pathnode.h"
#include "optimizer/planmain.h"
#include "optimizer/restrictinfo.h"
#include "optimizer/var.h"
#include "utils/memutils.h"
#include "utils/rel.h"

PG_MODULE_MAGIC;

/*
 * Describes the valid options for objects that use this wrapper.
 */
struct FileFdwOption
{
    const char *optname;
    Oid         optcontext;     /* Oid of catalog in which option may appear */
};

/*
 * Valid options for file_fdw.
 * These options are based on the options for COPY FROM command.
 * But note that force_not_null is handled as a boolean option attached to
 * each column, not as a table option.
 *
 * Note: If you are adding new option for user mapping, you need to modify
 * fileGetOptions(), which currently doesn't bother to look at user mappings.
 */
static const struct FileFdwOption valid_options[] = {
    /* File options */
    {"filename", ForeignTableRelationId},

    /* Format options */
    /* oids option is not supported */
    {"format", ForeignTableRelationId},
    {"header", ForeignTableRelationId},
    {"delimiter", ForeignTableRelationId},
    {"quote", ForeignTableRelationId},
    {"escape", ForeignTableRelationId},
    {"null", ForeignTableRelationId},
    {"encoding", ForeignTableRelationId},
    {"force_not_null", AttributeRelationId},

    /*
     * force_quote is not supported by file_fdw because it's for COPY TO.
     */

    /* Sentinel */
    {NULL, InvalidOid}
};

/*
 * FDW-specific information for RelOptInfo.fdw_private.
 */
typedef struct FileFdwPlanState
{
    char       *filename;       /* file to read */
    List       *options;        /* merged COPY options, excluding filename */
    BlockNumber pages;          /* estimate of file's physical size */
    double      ntuples;        /* estimate of number of rows in file */
} FileFdwPlanState;

/*
 * FDW-specific information for ForeignScanState.fdw_state.
 */
typedef struct FileFdwExecutionState
{
    char       *filename;       /* file to read */
    List       *options;        /* merged COPY options, excluding filename */
    CopyState   cstate;         /* state of reading file */
} FileFdwExecutionState;

/*
 * SQL functions
 */
extern Datum file_fdw_handler(PG_FUNCTION_ARGS);
extern Datum file_fdw_validator(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(file_fdw_handler);
PG_FUNCTION_INFO_V1(file_fdw_validator);

/*
 * FDW callback routines
 */
static void fileGetForeignRelSize(PlannerInfo *root,
                      RelOptInfo *baserel,
                      Oid foreigntableid);
static void fileGetForeignPaths(PlannerInfo *root,
                    RelOptInfo *baserel,
                    Oid foreigntableid);
static ForeignScan *fileGetForeignPlan(PlannerInfo *root,
                   RelOptInfo *baserel,
                   Oid foreigntableid,
                   ForeignPath *best_path,
                   List *tlist,
                   List *scan_clauses);
static void fileExplainForeignScan(ForeignScanState *node, ExplainState *es);
static void fileBeginForeignScan(ForeignScanState *node, int eflags);
static TupleTableSlot *fileIterateForeignScan(ForeignScanState *node);
static void fileReScanForeignScan(ForeignScanState *node);
static void fileEndForeignScan(ForeignScanState *node);
static bool fileAnalyzeForeignTable(Relation relation,
                        AcquireSampleRowsFunc *func,
                        BlockNumber *totalpages);

/*
 * Helper functions
 */
static bool is_valid_option(const char *option, Oid context);
static void fileGetOptions(Oid foreigntableid,
               char **filename, List **other_options);
static List *get_file_fdw_attribute_options(Oid relid);
static bool check_selective_binary_conversion(RelOptInfo *baserel,
                                              Oid foreigntableid,
                                              List **columns);
static void estimate_size(PlannerInfo *root, RelOptInfo *baserel,
              FileFdwPlanState *fdw_private);
static void estimate_costs(PlannerInfo *root, RelOptInfo *baserel,
               FileFdwPlanState *fdw_private,
               Cost *startup_cost, Cost *total_cost);
static int file_acquire_sample_rows(Relation onerel, int elevel,
                         HeapTuple *rows, int targrows,
                         double *totalrows, double *totaldeadrows);


/*
 * Foreign-data wrapper handler function: return a struct with pointers
 * to my callback routines.
 */
Datum
file_fdw_handler(PG_FUNCTION_ARGS)
{
    FdwRoutine *fdwroutine = makeNode(FdwRoutine);

    fdwroutine->GetForeignRelSize = fileGetForeignRelSize;
    fdwroutine->GetForeignPaths = fileGetForeignPaths;
    fdwroutine->GetForeignPlan = fileGetForeignPlan;
    fdwroutine->ExplainForeignScan = fileExplainForeignScan;
    fdwroutine->BeginForeignScan = fileBeginForeignScan;
    fdwroutine->IterateForeignScan = fileIterateForeignScan;
    fdwroutine->ReScanForeignScan = fileReScanForeignScan;
    fdwroutine->EndForeignScan = fileEndForeignScan;
    fdwroutine->AnalyzeForeignTable = fileAnalyzeForeignTable;

    PG_RETURN_POINTER(fdwroutine);
}

/*
 * Validate the generic options given to a FOREIGN DATA WRAPPER, SERVER,
 * USER MAPPING or FOREIGN TABLE that uses file_fdw.
 *
 * Raise an ERROR if the option or its value is considered invalid.
 */
Datum
file_fdw_validator(PG_FUNCTION_ARGS)
{
    List       *options_list = untransformRelOptions(PG_GETARG_DATUM(0));
    Oid         catalog = PG_GETARG_OID(1);
    char       *filename = NULL;
    DefElem    *force_not_null = NULL;
    List       *other_options = NIL;
    ListCell   *cell;

    /*
     * Only superusers are allowed to set options of a file_fdw foreign table.
     * This is because the filename is one of those options, and we don't want
     * non-superusers to be able to determine which file gets read.
     *
     * Putting this sort of permissions check in a validator is a bit of a
     * crock, but there doesn't seem to be any other place that can enforce
     * the check more cleanly.
     *
     * Note that the valid_options[] array disallows setting filename at any
     * options level other than foreign table --- otherwise there'd still be a
     * security hole.
     */
    if (catalog == ForeignTableRelationId && !superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("only superuser can change options of a file_fdw foreign table")));

    /*
     * Check that only options supported by file_fdw, and allowed for the
     * current object type, are given.
     */
    foreach(cell, options_list)
    {
        DefElem    *def = (DefElem *) lfirst(cell);

        if (!is_valid_option(def->defname, catalog))
        {
            const struct FileFdwOption *opt;
            StringInfoData buf;

            /*
             * Unknown option specified, complain about it. Provide a hint
             * with list of valid options for the object.
             */
            initStringInfo(&buf);
            for (opt = valid_options; opt->optname; opt++)
            {
                if (catalog == opt->optcontext)
                    appendStringInfo(&buf, "%s%s", (buf.len > 0) ? ", " : "",
                                     opt->optname);
            }

            ereport(ERROR,
                    (errcode(ERRCODE_FDW_INVALID_OPTION_NAME),
                     errmsg("invalid option \"%s\"", def->defname),
                     buf.len > 0
                     ? errhint("Valid options in this context are: %s",
                               buf.data)
                  : errhint("There are no valid options in this context.")));
        }

        /*
         * Separate out filename and force_not_null, since ProcessCopyOptions
         * won't accept them.  (force_not_null only comes in a boolean
         * per-column flavor here.)
         */
        if (strcmp(def->defname, "filename") == 0)
        {
            if (filename)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            filename = defGetString(def);
        }
        else if (strcmp(def->defname, "force_not_null") == 0)
        {
            if (force_not_null)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            force_not_null = def;
            /* Don't care what the value is, as long as it's a legal boolean */
            (void) defGetBoolean(def);
        }
        else
            other_options = lappend(other_options, def);
    }

    /*
     * Now apply the core COPY code's validation logic for more checks.
     */
    ProcessCopyOptions(NULL, true, other_options);

    /*
     * Filename option is required for file_fdw foreign tables.
     */
    if (catalog == ForeignTableRelationId && filename == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_FDW_DYNAMIC_PARAMETER_VALUE_NEEDED),
                 errmsg("filename is required for file_fdw foreign tables")));

    PG_RETURN_VOID();
}

/*
 * Check if the provided option is one of the valid options.
 * context is the Oid of the catalog holding the object the option is for.
 */
static bool
is_valid_option(const char *option, Oid context)
{
    const struct FileFdwOption *opt;

    for (opt = valid_options; opt->optname; opt++)
    {
        if (context == opt->optcontext && strcmp(opt->optname, option) == 0)
            return true;
    }
    return false;
}

/*
 * Fetch the options for a file_fdw foreign table.
 *
 * We have to separate out "filename" from the other options because
 * it must not appear in the options list passed to the core COPY code.
 */
static void
fileGetOptions(Oid foreigntableid,
               char **filename, List **other_options)
{
    ForeignTable *table;
    ForeignServer *server;
    ForeignDataWrapper *wrapper;
    List       *options;
    ListCell   *lc,
               *prev;

    /*
     * Extract options from FDW objects.  We ignore user mappings because
     * file_fdw doesn't have any options that can be specified there.
     *
     * (XXX Actually, given the current contents of valid_options[], there's
     * no point in examining anything except the foreign table's own options.
     * Simplify?)
     */
    table = GetForeignTable(foreigntableid);
    server = GetForeignServer(table->serverid);
    wrapper = GetForeignDataWrapper(server->fdwid);

    options = NIL;
    options = list_concat(options, wrapper->options);
    options = list_concat(options, server->options);
    options = list_concat(options, table->options);
    options = list_concat(options, get_file_fdw_attribute_options(foreigntableid));

    /*
     * Separate out the filename.
     */
    *filename = NULL;
    prev = NULL;
    foreach(lc, options)
    {
        DefElem    *def = (DefElem *) lfirst(lc);

        if (strcmp(def->defname, "filename") == 0)
        {
            *filename = defGetString(def);
            options = list_delete_cell(options, lc, prev);
            break;
        }
        prev = lc;
    }

    /*
     * The validator should have checked that a filename was included in the
     * options, but check again, just in case.
     */
    if (*filename == NULL)
        elog(ERROR, "filename is required for file_fdw foreign tables");

    *other_options = options;
}

/*
 * Retrieve per-column generic options from pg_attribute and construct a list
 * of DefElems representing them.
 *
 * At the moment we only have "force_not_null", which should be combined into
 * a single DefElem listing all such columns, since that's what COPY expects.
 */
static List *
get_file_fdw_attribute_options(Oid relid)
{
    Relation    rel;
    TupleDesc   tupleDesc;
    AttrNumber  natts;
    AttrNumber  attnum;
    List       *fnncolumns = NIL;

    rel = heap_open(relid, AccessShareLock);
    tupleDesc = RelationGetDescr(rel);
    natts = tupleDesc->natts;

    /* Retrieve FDW options for all user-defined attributes. */
    for (attnum = 1; attnum <= natts; attnum++)
    {
        Form_pg_attribute attr = tupleDesc->attrs[attnum - 1];
        List       *options;
        ListCell   *lc;

        /* Skip dropped attributes. */
        if (attr->attisdropped)
            continue;

        options = GetForeignColumnOptions(relid, attnum);
        foreach(lc, options)
        {
            DefElem    *def = (DefElem *) lfirst(lc);

            if (strcmp(def->defname, "force_not_null") == 0)
            {
                if (defGetBoolean(def))
                {
                    char       *attname = pstrdup(NameStr(attr->attname));

                    fnncolumns = lappend(fnncolumns, makeString(attname));
                }
            }
            /* maybe in future handle other options here */
        }
    }

    heap_close(rel, AccessShareLock);

    /* Return DefElem only when some column(s) have force_not_null */
    if (fnncolumns != NIL)
        return list_make1(makeDefElem("force_not_null", (Node *) fnncolumns));
    else
        return NIL;
}

/*
 * fileGetForeignRelSize
 *      Obtain relation size estimates for a foreign table
 */
static void
fileGetForeignRelSize(PlannerInfo *root,
                      RelOptInfo *baserel,
                      Oid foreigntableid)
{
    FileFdwPlanState *fdw_private;

    /*
     * Fetch options.  We only need filename at this point, but we might as
     * well get everything and not need to re-fetch it later in planning.
     */
    fdw_private = (FileFdwPlanState *) palloc(sizeof(FileFdwPlanState));
    fileGetOptions(foreigntableid,
                   &fdw_private->filename, &fdw_private->options);
    baserel->fdw_private = (void *) fdw_private;

    /* Estimate relation size */
    estimate_size(root, baserel, fdw_private);
}

/*
 * fileGetForeignPaths
 *      Create possible access paths for a scan on the foreign table
 *
 *      Currently we don't support any push-down feature, so there is only one
 *      possible access path, which simply returns all records in the order in
 *      the data file.
 */
static void
fileGetForeignPaths(PlannerInfo *root,
                    RelOptInfo *baserel,
                    Oid foreigntableid)
{
    FileFdwPlanState *fdw_private = (FileFdwPlanState *) baserel->fdw_private;
    Cost        startup_cost;
    Cost        total_cost;
    List       *columns;
    List       *coptions = NIL;

    /* Decide whether to selectively perform binary conversion */
    if (check_selective_binary_conversion(baserel,
                                          foreigntableid,
                                          &columns))
        coptions = list_make1(makeDefElem("convert_selectively",
                                          (Node *) columns));

    /* Estimate costs */
    estimate_costs(root, baserel, fdw_private,
                   &startup_cost, &total_cost);

    /*
     * Create a ForeignPath node and add it as only possible path.  We use the
     * fdw_private list of the path to carry the convert_selectively option;
     * it will be propagated into the fdw_private list of the Plan node.
     */
    add_path(baserel, (Path *)
             create_foreignscan_path(root, baserel,
                                     baserel->rows,
                                     startup_cost,
                                     total_cost,
                                     NIL,       /* no pathkeys */
                                     NULL,      /* no outer rel either */
                                     coptions));

    /*
     * If data file was sorted, and we knew it somehow, we could insert
     * appropriate pathkeys into the ForeignPath node to tell the planner
     * that.
     */
}

/*
 * fileGetForeignPlan
 *      Create a ForeignScan plan node for scanning the foreign table
 */
static ForeignScan *
fileGetForeignPlan(PlannerInfo *root,
                   RelOptInfo *baserel,
                   Oid foreigntableid,
                   ForeignPath *best_path,
                   List *tlist,
                   List *scan_clauses)
{
    Index       scan_relid = baserel->relid;

    /*
     * We have no native ability to evaluate restriction clauses, so we just
     * put all the scan_clauses into the plan node's qual list for the
     * executor to check.  So all we have to do here is strip RestrictInfo
     * nodes from the clauses and ignore pseudoconstants (which will be
     * handled elsewhere).
     */
    scan_clauses = extract_actual_clauses(scan_clauses, false);

    /* Create the ForeignScan node */
    return make_foreignscan(tlist,
                            scan_clauses,
                            scan_relid,
                            NIL,    /* no expressions to evaluate */
                            best_path->fdw_private);
}

/*
 * fileExplainForeignScan
 *      Produce extra output for EXPLAIN
 */
static void
fileExplainForeignScan(ForeignScanState *node, ExplainState *es)
{
    char       *filename;
    List       *options;

    /* Fetch options --- we only need filename at this point */
    fileGetOptions(RelationGetRelid(node->ss.ss_currentRelation),
                   &filename, &options);

    ExplainPropertyText("Foreign File", filename, es);

    /* Suppress file size if we're not showing cost details */
    if (es->costs)
    {
        struct stat stat_buf;

        if (stat(filename, &stat_buf) == 0)
            ExplainPropertyLong("Foreign File Size", (long) stat_buf.st_size,
                                es);
    }
}

/*
 * fileBeginForeignScan
 *      Initiate access to the file by creating CopyState
 */
static void
fileBeginForeignScan(ForeignScanState *node, int eflags)
{
    ForeignScan *plan = (ForeignScan *) node->ss.ps.plan;
    char       *filename;
    List       *options;
    CopyState   cstate;
    FileFdwExecutionState *festate;

    /*
     * Do nothing in EXPLAIN (no ANALYZE) case.  node->fdw_state stays NULL.
     */
    if (eflags & EXEC_FLAG_EXPLAIN_ONLY)
        return;

    /* Fetch options of foreign table */
    fileGetOptions(RelationGetRelid(node->ss.ss_currentRelation),
                   &filename, &options);

    /* Add any options from the plan (currently only convert_selectively) */
    options = list_concat(options, plan->fdw_private);

    /*
     * Create CopyState from FDW options.  We always acquire all columns, so
     * as to match the expected ScanTupleSlot signature.
     */
    cstate = BeginCopyFrom(node->ss.ss_currentRelation,
                           filename,
                           false,
                           NIL,
                           options);

    /*
     * Save state in node->fdw_state.  We must save enough information to call
     * BeginCopyFrom() again.
     */
    festate = (FileFdwExecutionState *) palloc(sizeof(FileFdwExecutionState));
    festate->filename = filename;
    festate->options = options;
    festate->cstate = cstate;

    node->fdw_state = (void *) festate;
}

/*
 * fileIterateForeignScan
 *      Read next record from the data file and store it into the
 *      ScanTupleSlot as a virtual tuple
 */
static TupleTableSlot *
fileIterateForeignScan(ForeignScanState *node)
{
    FileFdwExecutionState *festate = (FileFdwExecutionState *) node->fdw_state;
    TupleTableSlot *slot = node->ss.ss_ScanTupleSlot;
    bool        found;
    ErrorContextCallback errcallback;

    /* Set up callback to identify error line number. */
    errcallback.callback = CopyFromErrorCallback;
    errcallback.arg = (void *) festate->cstate;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    /*
     * The protocol for loading a virtual tuple into a slot is first
     * ExecClearTuple, then fill the values/isnull arrays, then
     * ExecStoreVirtualTuple.  If we don't find another row in the file, we
     * just skip the last step, leaving the slot empty as required.
     *
     * We can pass ExprContext = NULL because we read all columns from the
     * file, so no need to evaluate default expressions.
     *
     * We can also pass tupleOid = NULL because we don't allow oids for
     * foreign tables.
     */
    ExecClearTuple(slot);
    found = NextCopyFrom(festate->cstate, NULL,
                         slot->tts_values, slot->tts_isnull,
                         NULL);
    if (found)
        ExecStoreVirtualTuple(slot);

    /* Remove error callback. */
    error_context_stack = errcallback.previous;

    return slot;
}

/*
 * fileReScanForeignScan
 *      Rescan table, possibly with new parameters
 */
static void
fileReScanForeignScan(ForeignScanState *node)
{
    FileFdwExecutionState *festate = (FileFdwExecutionState *) node->fdw_state;

    EndCopyFrom(festate->cstate);

    festate->cstate = BeginCopyFrom(node->ss.ss_currentRelation,
                                    festate->filename,
                                    false,
                                    NIL,
                                    festate->options);
}

/*
 * fileEndForeignScan
 *      Finish scanning foreign table and dispose objects used for this scan
 */
static void
fileEndForeignScan(ForeignScanState *node)
{
    FileFdwExecutionState *festate = (FileFdwExecutionState *) node->fdw_state;

    /* if festate is NULL, we are in EXPLAIN; nothing to do */
    if (festate)
        EndCopyFrom(festate->cstate);
}

/*
 * fileAnalyzeForeignTable
 *      Test whether analyzing this foreign table is supported
 */
static bool
fileAnalyzeForeignTable(Relation relation,
                        AcquireSampleRowsFunc *func,
                        BlockNumber *totalpages)
{
    char       *filename;
    List       *options;
    struct stat stat_buf;

    /* Fetch options of foreign table */
    fileGetOptions(RelationGetRelid(relation), &filename, &options);

    /*
     * Get size of the file.  (XXX if we fail here, would it be better to just
     * return false to skip analyzing the table?)
     */
    if (stat(filename, &stat_buf) < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not stat file \"%s\": %m",
                        filename)));

    /*
     * Convert size to pages.  Must return at least 1 so that we can tell
     * later on that pg_class.relpages is not default.
     */
    *totalpages = (stat_buf.st_size + (BLCKSZ - 1)) / BLCKSZ;
    if (*totalpages < 1)
        *totalpages = 1;

    *func = file_acquire_sample_rows;

    return true;
}

/*
 * check_selective_binary_conversion
 *
 * Check to see if it's useful to convert only a subset of the file's columns
 * to binary.  If so, construct a list of the column names to be converted,
 * return that at *columns, and return TRUE.  (Note that it's possible to
 * determine that no columns need be converted, for instance with a COUNT(*)
 * query.  So we can't use returning a NIL list to indicate failure.)
 */
static bool
check_selective_binary_conversion(RelOptInfo *baserel,
                                  Oid foreigntableid,
                                  List **columns)
{
    ForeignTable *table;
    ListCell   *lc;
    Relation    rel;
    TupleDesc   tupleDesc;
    AttrNumber  attnum;
    Bitmapset  *attrs_used = NULL;
    bool        has_wholerow = false;
    int         numattrs;
    int         i;

    *columns = NIL;             /* default result */

    /*
     * Check format of the file.  If binary format, this is irrelevant.
     */
    table = GetForeignTable(foreigntableid);
    foreach(lc, table->options)
    {
        DefElem    *def = (DefElem *) lfirst(lc);

        if (strcmp(def->defname, "format") == 0)
        {
            char       *format = defGetString(def);

            if (strcmp(format, "binary") == 0)
                return false;
            break;
        }
    }

    /* Collect all the attributes needed for joins or final output. */
    pull_varattnos((Node *) baserel->reltargetlist, baserel->relid,
                   &attrs_used);

    /* Add all the attributes used by restriction clauses. */
    foreach(lc, baserel->baserestrictinfo)
    {
        RestrictInfo   *rinfo = (RestrictInfo *) lfirst(lc);

        pull_varattnos((Node *) rinfo->clause, baserel->relid,
                       &attrs_used);
    }

    /* Convert attribute numbers to column names. */
    rel = heap_open(foreigntableid, AccessShareLock);
    tupleDesc = RelationGetDescr(rel);

    while ((attnum = bms_first_member(attrs_used)) >= 0)
    {
        /* Adjust for system attributes. */
        attnum += FirstLowInvalidHeapAttributeNumber;

        if (attnum == 0)
        {
            has_wholerow = true;
            break;
        }

        /* Ignore system attributes. */
        if (attnum < 0)
            continue;

        /* Get user attributes. */
        if (attnum > 0)
        {
            Form_pg_attribute attr = tupleDesc->attrs[attnum - 1];
            char       *attname = NameStr(attr->attname);

            /* Skip dropped attributes (probably shouldn't see any here). */
            if (attr->attisdropped)
                continue;
            *columns = lappend(*columns, makeString(pstrdup(attname)));
        }
    }

    /* Count non-dropped user attributes while we have the tupdesc. */
    numattrs = 0;
    for (i = 0; i < tupleDesc->natts; i++)
    {
        Form_pg_attribute attr = tupleDesc->attrs[i];

        if (attr->attisdropped)
            continue;
        numattrs++;
    }

    heap_close(rel, AccessShareLock);

    /* If there's a whole-row reference, fail: we need all the columns. */
    if (has_wholerow)
    {
        *columns = NIL;
        return false;
    }

    /* If all the user attributes are needed, fail. */
    if (numattrs == list_length(*columns))
    {
        *columns = NIL;
        return false;
    }

    return true;
}

/*
 * Estimate size of a foreign table.
 *
 * The main result is returned in baserel->rows.  We also set
 * fdw_private->pages and fdw_private->ntuples for later use in the cost
 * calculation.
 */
static void
estimate_size(PlannerInfo *root, RelOptInfo *baserel,
              FileFdwPlanState *fdw_private)
{
    struct stat stat_buf;
    BlockNumber pages;
    double      ntuples;
    double      nrows;

    /*
     * Get size of the file.  It might not be there at plan time, though, in
     * which case we have to use a default estimate.
     */
    if (stat(fdw_private->filename, &stat_buf) < 0)
        stat_buf.st_size = 10 * BLCKSZ;

    /*
     * Convert size to pages for use in I/O cost estimate later.
     */
    pages = (stat_buf.st_size + (BLCKSZ - 1)) / BLCKSZ;
    if (pages < 1)
        pages = 1;
    fdw_private->pages = pages;

    /*
     * Estimate the number of tuples in the file.
     */
    if (baserel->pages > 0)
    {
        /*
         * We have # of pages and # of tuples from pg_class (that is, from a
         * previous ANALYZE), so compute a tuples-per-page estimate and scale
         * that by the current file size.
         */
        double      density;

        density = baserel->tuples / (double) baserel->pages;
        ntuples = clamp_row_est(density * (double) pages);
    }
    else
    {
        /*
         * Otherwise we have to fake it.  We back into this estimate using the
         * planner's idea of the relation width; which is bogus if not all
         * columns are being read, not to mention that the text representation
         * of a row probably isn't the same size as its internal
         * representation.  Possibly we could do something better, but the
         * real answer to anyone who complains is "ANALYZE" ...
         */
        int         tuple_width;

        tuple_width = MAXALIGN(baserel->width) +
            MAXALIGN(sizeof(HeapTupleHeaderData));
        ntuples = clamp_row_est((double) stat_buf.st_size /
                                (double) tuple_width);
    }
    fdw_private->ntuples = ntuples;

    /*
     * Now estimate the number of rows returned by the scan after applying the
     * baserestrictinfo quals.
     */
    nrows = ntuples *
        clauselist_selectivity(root,
                               baserel->baserestrictinfo,
                               0,
                               JOIN_INNER,
                               NULL);

    nrows = clamp_row_est(nrows);

    /* Save the output-rows estimate for the planner */
    baserel->rows = nrows;
}

/*
 * Estimate costs of scanning a foreign table.
 *
 * Results are returned in *startup_cost and *total_cost.
 */
static void
estimate_costs(PlannerInfo *root, RelOptInfo *baserel,
               FileFdwPlanState *fdw_private,
               Cost *startup_cost, Cost *total_cost)
{
    BlockNumber pages = fdw_private->pages;
    double      ntuples = fdw_private->ntuples;
    Cost        run_cost = 0;
    Cost        cpu_per_tuple;

    /*
     * We estimate costs almost the same way as cost_seqscan(), thus assuming
     * that I/O costs are equivalent to a regular table file of the same size.
     * However, we take per-tuple CPU costs as 10x of a seqscan, to account
     * for the cost of parsing records.
     */
    run_cost += seq_page_cost * pages;

    *startup_cost = baserel->baserestrictcost.startup;
    cpu_per_tuple = cpu_tuple_cost * 10 + baserel->baserestrictcost.per_tuple;
    run_cost += cpu_per_tuple * ntuples;
    *total_cost = *startup_cost + run_cost;
}

/*
 * file_acquire_sample_rows -- acquire a random sample of rows from the table
 *
 * Selected rows are returned in the caller-allocated array rows[],
 * which must have at least targrows entries.
 * The actual number of rows selected is returned as the function result.
 * We also count the total number of rows in the file and return it into
 * *totalrows.  Note that *totaldeadrows is always set to 0.
 *
 * Note that the returned list of rows is not always in order by physical
 * position in the file.  Therefore, correlation estimates derived later
 * may be meaningless, but it's OK because we don't use the estimates
 * currently (the planner only pays attention to correlation for indexscans).
 */
static int
file_acquire_sample_rows(Relation onerel, int elevel,
                         HeapTuple *rows, int targrows,
                         double *totalrows, double *totaldeadrows)
{
    int         numrows = 0;
    double      rowstoskip = -1;    /* -1 means not set yet */
    double      rstate;
    TupleDesc   tupDesc;
    Datum      *values;
    bool       *nulls;
    bool        found;
    char       *filename;
    List       *options;
    CopyState   cstate;
    ErrorContextCallback errcallback;
    MemoryContext oldcontext = CurrentMemoryContext;
    MemoryContext tupcontext;

    Assert(onerel);
    Assert(targrows > 0);

    tupDesc = RelationGetDescr(onerel);
    values = (Datum *) palloc(tupDesc->natts * sizeof(Datum));
    nulls = (bool *) palloc(tupDesc->natts * sizeof(bool));

    /* Fetch options of foreign table */
    fileGetOptions(RelationGetRelid(onerel), &filename, &options);

    /*
     * Create CopyState from FDW options.
     */
    cstate = BeginCopyFrom(onerel, filename, false, NIL, options);

    /*
     * Use per-tuple memory context to prevent leak of memory used to read
     * rows from the file with Copy routines.
     */
    tupcontext = AllocSetContextCreate(CurrentMemoryContext,
                                       "file_fdw temporary context",
                                       ALLOCSET_DEFAULT_MINSIZE,
                                       ALLOCSET_DEFAULT_INITSIZE,
                                       ALLOCSET_DEFAULT_MAXSIZE);

    /* Prepare for sampling rows */
    rstate = anl_init_selection_state(targrows);

    /* Set up callback to identify error line number. */
    errcallback.callback = CopyFromErrorCallback;
    errcallback.arg = (void *) cstate;
    errcallback.previous = error_context_stack;
    error_context_stack = &errcallback;

    *totalrows = 0;
    *totaldeadrows = 0;
    for (;;)
    {
        /* Check for user-requested abort or sleep */
        vacuum_delay_point();

        /* Fetch next row */
        MemoryContextReset(tupcontext);
        MemoryContextSwitchTo(tupcontext);

        found = NextCopyFrom(cstate, NULL, values, nulls, NULL);

        MemoryContextSwitchTo(oldcontext);

        if (!found)
            break;

        /*
         * The first targrows sample rows are simply copied into the
         * reservoir.  Then we start replacing tuples in the sample until we
         * reach the end of the relation. This algorithm is from Jeff Vitter's
         * paper (see more info in commands/analyze.c).
         */
        if (numrows < targrows)
        {
            rows[numrows++] = heap_form_tuple(tupDesc, values, nulls);
        }
        else
        {
            /*
             * t in Vitter's paper is the number of records already processed.
             * If we need to compute a new S value, we must use the
             * not-yet-incremented value of totalrows as t.
             */
            if (rowstoskip < 0)
                rowstoskip = anl_get_next_S(*totalrows, targrows, &rstate);

            if (rowstoskip <= 0)
            {
                /*
                 * Found a suitable tuple, so save it, replacing one old tuple
                 * at random
                 */
                int         k = (int) (targrows * anl_random_fract());

                Assert(k >= 0 && k < targrows);
                heap_freetuple(rows[k]);
                rows[k] = heap_form_tuple(tupDesc, values, nulls);
            }

            rowstoskip -= 1;
        }

        *totalrows += 1;
    }

    /* Remove error callback. */
    error_context_stack = errcallback.previous;

    /* Clean up. */
    MemoryContextDelete(tupcontext);

    EndCopyFrom(cstate);

    pfree(values);
    pfree(nulls);

    /*
     * Emit some interesting relation info
     */
    ereport(elevel,
            (errmsg("\"%s\": file contains %.0f rows; "
                    "%d rows in sample",
                    RelationGetRelationName(onerel),
                    *totalrows, numrows)));

    return numrows;
}