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foreign_grouping_ok() is willing to put fairly arbitrary expressions into
the targetlist of a remote SELECT that's doing grouping or aggregation on
the remote side, including expressions that have no foreign component to
them at all. This is possibly a bit dubious from an efficiency standpoint;
but it rises to the level of a crash-causing bug if the expression is just
a Param or non-foreign Var. In that case, the expression will necessarily
also appear in the fdw_exprs list of values we need to send to the remote
server, and then setrefs.c's set_foreignscan_references will mistakenly
replace the fdw_exprs entry with a Var referencing the targetlist result.
The root cause of this problem is bad design in commit e7cb7ee14: it put
logic into set_foreignscan_references that IMV is postgres_fdw-specific,
and yet this bug shows that it isn't postgres_fdw-specific enough. The
transformation being done on fdw_exprs assumes that fdw_exprs is to be
evaluated with the fdw_scan_tlist as input, which is not how postgres_fdw
uses it; yet it could be the right thing for some other FDW. (In the
bigger picture, setrefs.c has no business assuming this for the other
expression fields of a ForeignScan either.)
The right fix therefore would be to expand the FDW API so that the
FDW could inform setrefs.c how it intends to evaluate these various
expressions. We can't change that in the back branches though, and we
also can't just summarily change setrefs.c's behavior there, or we're
likely to break external FDWs.
As a stopgap, therefore, hack up postgres_fdw so that it won't attempt
to send targetlist entries that look exactly like the fdw_exprs entries
they'd produce. In most cases this actually produces a superior plan,
IMO, with less data needing to be transmitted and returned; so we probably
ought to think harder about whether we should ship tlist expressions at
all when they don't contain any foreign Vars or Aggs. But that's an
optimization not a bug fix so I left it for later. One case where this
produces an inferior plan is where the expression in question is actually
a GROUP BY expression: then the restriction prevents us from using remote
grouping. It might be possible to work around that (since that would
reduce to group-by-a-constant on the remote side); but it seems like a
pretty unlikely corner case, so I'm not sure it's worth expending effort
solely to improve that. In any case the right long-term answer is to fix
the API as sketched above, and then revert this hack.
Per bug #15781 from Sean Johnston. Back-patch to v10 where the problem
was introduced.
Discussion: https://postgr.es/m/15781-2601b1002bad087c@postgresql.org
191 lines
6.3 KiB
C
191 lines
6.3 KiB
C
/*-------------------------------------------------------------------------
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*
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* postgres_fdw.h
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* Foreign-data wrapper for remote PostgreSQL servers
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*
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* Portions Copyright (c) 2012-2018, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* contrib/postgres_fdw/postgres_fdw.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef POSTGRES_FDW_H
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#define POSTGRES_FDW_H
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#include "foreign/foreign.h"
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#include "lib/stringinfo.h"
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#include "nodes/relation.h"
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#include "utils/relcache.h"
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#include "libpq-fe.h"
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/*
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* FDW-specific planner information kept in RelOptInfo.fdw_private for a
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* postgres_fdw foreign table. For a baserel, this struct is created by
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* postgresGetForeignRelSize, although some fields are not filled till later.
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* postgresGetForeignJoinPaths creates it for a joinrel, and
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* postgresGetForeignUpperPaths creates it for an upperrel.
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*/
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typedef struct PgFdwRelationInfo
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{
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/*
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* True means that the relation can be pushed down. Always true for simple
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* foreign scan.
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*/
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bool pushdown_safe;
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/*
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* Restriction clauses, divided into safe and unsafe to pushdown subsets.
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* All entries in these lists should have RestrictInfo wrappers; that
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* improves efficiency of selectivity and cost estimation.
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*/
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List *remote_conds;
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List *local_conds;
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/* Actual remote restriction clauses for scan (sans RestrictInfos) */
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List *final_remote_exprs;
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/* Bitmap of attr numbers we need to fetch from the remote server. */
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Bitmapset *attrs_used;
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/* Cost and selectivity of local_conds. */
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QualCost local_conds_cost;
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Selectivity local_conds_sel;
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/* Selectivity of join conditions */
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Selectivity joinclause_sel;
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/* Estimated size and cost for a scan or join. */
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double rows;
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int width;
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Cost startup_cost;
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Cost total_cost;
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/* Costs excluding costs for transferring data from the foreign server */
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Cost rel_startup_cost;
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Cost rel_total_cost;
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/* Options extracted from catalogs. */
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bool use_remote_estimate;
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Cost fdw_startup_cost;
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Cost fdw_tuple_cost;
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List *shippable_extensions; /* OIDs of whitelisted extensions */
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/* Cached catalog information. */
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ForeignTable *table;
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ForeignServer *server;
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UserMapping *user; /* only set in use_remote_estimate mode */
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int fetch_size; /* fetch size for this remote table */
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/*
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* Name of the relation while EXPLAINing ForeignScan. It is used for join
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* relations but is set for all relations. For join relation, the name
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* indicates which foreign tables are being joined and the join type used.
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*/
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StringInfo relation_name;
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/* Join information */
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RelOptInfo *outerrel;
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RelOptInfo *innerrel;
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JoinType jointype;
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/* joinclauses contains only JOIN/ON conditions for an outer join */
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List *joinclauses; /* List of RestrictInfo */
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/* Grouping information */
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List *grouped_tlist;
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/* Subquery information */
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bool make_outerrel_subquery; /* do we deparse outerrel as a
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* subquery? */
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bool make_innerrel_subquery; /* do we deparse innerrel as a
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* subquery? */
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Relids lower_subquery_rels; /* all relids appearing in lower
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* subqueries */
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/*
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* Index of the relation. It is used to create an alias to a subquery
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* representing the relation.
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*/
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int relation_index;
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} PgFdwRelationInfo;
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/* in postgres_fdw.c */
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extern int set_transmission_modes(void);
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extern void reset_transmission_modes(int nestlevel);
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/* in connection.c */
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extern PGconn *GetConnection(UserMapping *user, bool will_prep_stmt);
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extern void ReleaseConnection(PGconn *conn);
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extern unsigned int GetCursorNumber(PGconn *conn);
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extern unsigned int GetPrepStmtNumber(PGconn *conn);
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extern PGresult *pgfdw_get_result(PGconn *conn, const char *query);
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extern PGresult *pgfdw_exec_query(PGconn *conn, const char *query);
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extern void pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
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bool clear, const char *sql);
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/* in option.c */
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extern int ExtractConnectionOptions(List *defelems,
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const char **keywords,
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const char **values);
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extern List *ExtractExtensionList(const char *extensionsString,
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bool warnOnMissing);
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/* in deparse.c */
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extern void classifyConditions(PlannerInfo *root,
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RelOptInfo *baserel,
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List *input_conds,
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List **remote_conds,
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List **local_conds);
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extern bool is_foreign_expr(PlannerInfo *root,
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RelOptInfo *baserel,
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Expr *expr);
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extern bool is_foreign_param(PlannerInfo *root,
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RelOptInfo *baserel,
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Expr *expr);
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extern void deparseInsertSql(StringInfo buf, RangeTblEntry *rte,
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Index rtindex, Relation rel,
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List *targetAttrs, bool doNothing, List *returningList,
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List **retrieved_attrs);
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extern void deparseUpdateSql(StringInfo buf, RangeTblEntry *rte,
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Index rtindex, Relation rel,
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List *targetAttrs, List *returningList,
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List **retrieved_attrs);
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extern void deparseDirectUpdateSql(StringInfo buf, PlannerInfo *root,
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Index rtindex, Relation rel,
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RelOptInfo *foreignrel,
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List *targetlist,
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List *targetAttrs,
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List *remote_conds,
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List **params_list,
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List *returningList,
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List **retrieved_attrs);
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extern void deparseDeleteSql(StringInfo buf, RangeTblEntry *rte,
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Index rtindex, Relation rel,
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List *returningList,
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List **retrieved_attrs);
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extern void deparseDirectDeleteSql(StringInfo buf, PlannerInfo *root,
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Index rtindex, Relation rel,
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RelOptInfo *foreignrel,
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List *remote_conds,
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List **params_list,
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List *returningList,
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List **retrieved_attrs);
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extern void deparseAnalyzeSizeSql(StringInfo buf, Relation rel);
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extern void deparseAnalyzeSql(StringInfo buf, Relation rel,
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List **retrieved_attrs);
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extern void deparseStringLiteral(StringInfo buf, const char *val);
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extern Expr *find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel);
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extern List *build_tlist_to_deparse(RelOptInfo *foreignrel);
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extern void deparseSelectStmtForRel(StringInfo buf, PlannerInfo *root,
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RelOptInfo *foreignrel, List *tlist,
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List *remote_conds, List *pathkeys, bool is_subquery,
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List **retrieved_attrs, List **params_list);
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extern const char *get_jointype_name(JoinType jointype);
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/* in shippable.c */
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extern bool is_builtin(Oid objectId);
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extern bool is_shippable(Oid objectId, Oid classId, PgFdwRelationInfo *fpinfo);
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#endif /* POSTGRES_FDW_H */
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