portal.h

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/*-------------------------------------------------------------------------
 *
 * portal.h
 *    POSTGRES portal definitions.
 *
 * A portal is an abstraction which represents the execution state of
 * a running or runnable query.  Portals support both SQL-level CURSORs
 * and protocol-level portals.
 *
 * Scrolling (nonsequential access) and suspension of execution are allowed
 * only for portals that contain a single SELECT-type query.  We do not want
 * to let the client suspend an update-type query partway through!  Because
 * the query rewriter does not allow arbitrary ON SELECT rewrite rules,
 * only queries that were originally update-type could produce multiple
 * plan trees; so the restriction to a single query is not a problem
 * in practice.
 *
 * For SQL cursors, we support three kinds of scroll behavior:
 *
 * (1) Neither NO SCROLL nor SCROLL was specified: to remain backward
 *     compatible, we allow backward fetches here, unless it would
 *     impose additional runtime overhead to do so.
 *
 * (2) NO SCROLL was specified: don't allow any backward fetches.
 *
 * (3) SCROLL was specified: allow all kinds of backward fetches, even
 *     if we need to take a performance hit to do so.  (The planner sticks
 *     a Materialize node atop the query plan if needed.)
 *
 * Case #1 is converted to #2 or #3 by looking at the query itself and
 * determining if scrollability can be supported without additional
 * overhead.
 *
 * Protocol-level portals have no nonsequential-fetch API and so the
 * distinction doesn't matter for them.  They are always initialized
 * to look like NO SCROLL cursors.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/utils/portal.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef PORTAL_H
#define PORTAL_H

#include "datatype/timestamp.h"
#include "executor/execdesc.h"
#include "tcop/cmdtag.h"
#include "utils/plancache.h"
#include "utils/resowner.h"

/*
 * We have several execution strategies for Portals, depending on what
 * query or queries are to be executed.  (Note: in all cases, a Portal
 * executes just a single source-SQL query, and thus produces just a
 * single result from the user's viewpoint.  However, the rule rewriter
 * may expand the single source query to zero or many actual queries.)
 *
 * PORTAL_ONE_SELECT: the portal contains one single SELECT query.  We run
 * the Executor incrementally as results are demanded.  This strategy also
 * supports holdable cursors (the Executor results can be dumped into a
 * tuplestore for access after transaction completion).
 *
 * PORTAL_ONE_RETURNING: the portal contains a single INSERT/UPDATE/DELETE
 * query with a RETURNING clause (plus possibly auxiliary queries added by
 * rule rewriting).  On first execution, we run the portal to completion
 * and dump the primary query's results into the portal tuplestore; the
 * results are then returned to the client as demanded.  (We can't support
 * suspension of the query partway through, because the AFTER TRIGGER code
 * can't cope, and also because we don't want to risk failing to execute
 * all the auxiliary queries.)
 *
 * PORTAL_ONE_MOD_WITH: the portal contains one single SELECT query, but
 * it has data-modifying CTEs.  This is currently treated the same as the
 * PORTAL_ONE_RETURNING case because of the possibility of needing to fire
 * triggers.  It may act more like PORTAL_ONE_SELECT in future.
 *
 * PORTAL_UTIL_SELECT: the portal contains a utility statement that returns
 * a SELECT-like result (for example, EXPLAIN or SHOW).  On first execution,
 * we run the statement and dump its results into the portal tuplestore;
 * the results are then returned to the client as demanded.
 *
 * PORTAL_MULTI_QUERY: all other cases.  Here, we do not support partial
 * execution: the portal's queries will be run to completion on first call.
 */
typedef enum PortalStrategy
{
    PORTAL_ONE_SELECT,
    PORTAL_ONE_RETURNING,
    PORTAL_ONE_MOD_WITH,
    PORTAL_UTIL_SELECT,
    PORTAL_MULTI_QUERY
} PortalStrategy;

/*
 * A portal is always in one of these states.  It is possible to transit
 * from ACTIVE back to READY if the query is not run to completion;
 * otherwise we never back up in status.
 */
typedef enum PortalStatus
{
    PORTAL_NEW,                 /* freshly created */
    PORTAL_DEFINED,             /* PortalDefineQuery done */
    PORTAL_READY,               /* PortalStart complete, can run it */
    PORTAL_ACTIVE,              /* portal is running (can't delete it) */
    PORTAL_DONE,                /* portal is finished (don't re-run it) */
    PORTAL_FAILED               /* portal got error (can't re-run it) */
} PortalStatus;

typedef struct PortalData *Portal;

typedef struct PortalData
{
    /* Bookkeeping data */
    const char *name;           /* portal's name */
    const char *prepStmtName;   /* source prepared statement (NULL if none) */
    MemoryContext portalContext;    /* subsidiary memory for portal */
    ResourceOwner resowner;     /* resources owned by portal */
    void        (*cleanup) (Portal portal); /* cleanup hook */

    /*
     * State data for remembering which subtransaction(s) the portal was
     * created or used in.  If the portal is held over from a previous
     * transaction, both subxids are InvalidSubTransactionId.  Otherwise,
     * createSubid is the creating subxact and activeSubid is the last subxact
     * in which we ran the portal.
     */
    SubTransactionId createSubid;   /* the creating subxact */
    SubTransactionId activeSubid;   /* the last subxact with activity */

    /* The query or queries the portal will execute */
    const char *sourceText;     /* text of query (as of 8.4, never NULL) */
    CommandTag  commandTag;     /* command tag for original query */
    QueryCompletion qc;         /* command completion data for executed query */
    List       *stmts;          /* list of PlannedStmts */
    CachedPlan *cplan;          /* CachedPlan, if stmts are from one */

    ParamListInfo portalParams; /* params to pass to query */
    QueryEnvironment *queryEnv; /* environment for query */

    /* Features/options */
    PortalStrategy strategy;    /* see above */
    int         cursorOptions;  /* DECLARE CURSOR option bits */
    bool        run_once;       /* portal will only be run once */

    /* Status data */
    PortalStatus status;        /* see above */
    bool        portalPinned;   /* a pinned portal can't be dropped */
    bool        autoHeld;       /* was automatically converted from pinned to
                                 * held (see HoldPinnedPortals()) */

    /* If not NULL, Executor is active; call ExecutorEnd eventually: */
    QueryDesc  *queryDesc;      /* info needed for executor invocation */

    /* If portal returns tuples, this is their tupdesc: */
    TupleDesc   tupDesc;        /* descriptor for result tuples */
    /* and these are the format codes to use for the columns: */
    int16      *formats;        /* a format code for each column */

    /*
     * Outermost ActiveSnapshot for execution of the portal's queries.  For
     * all but a few utility commands, we require such a snapshot to exist.
     * This ensures that TOAST references in query results can be detoasted,
     * and helps to reduce thrashing of the process's exposed xmin.
     */
    Snapshot    portalSnapshot; /* active snapshot, or NULL if none */

    /*
     * Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING or
     * PORTAL_UTIL_SELECT query.  (A cursor held past the end of its
     * transaction no longer has any active executor state.)
     */
    Tuplestorestate *holdStore; /* store for holdable cursors */
    MemoryContext holdContext;  /* memory containing holdStore */

    /*
     * Snapshot under which tuples in the holdStore were read.  We must keep a
     * reference to this snapshot if there is any possibility that the tuples
     * contain TOAST references, because releasing the snapshot could allow
     * recently-dead rows to be vacuumed away, along with any toast data
     * belonging to them.  In the case of a held cursor, we avoid needing to
     * keep such a snapshot by forcibly detoasting the data.
     */
    Snapshot    holdSnapshot;   /* registered snapshot, or NULL if none */

    /*
     * atStart, atEnd and portalPos indicate the current cursor position.
     * portalPos is zero before the first row, N after fetching N'th row of
     * query.  After we run off the end, portalPos = # of rows in query, and
     * atEnd is true.  Note that atStart implies portalPos == 0, but not the
     * reverse: we might have backed up only as far as the first row, not to
     * the start.  Also note that various code inspects atStart and atEnd, but
     * only the portal movement routines should touch portalPos.
     */
    bool        atStart;
    bool        atEnd;
    uint64      portalPos;

    /* Presentation data, primarily used by the pg_cursors system view */
    TimestampTz creation_time;  /* time at which this portal was defined */
    bool        visible;        /* include this portal in pg_cursors? */
}           PortalData;

/*
 * PortalIsValid
 *      True iff portal is valid.
 */
#define PortalIsValid(p) PointerIsValid(p)


/* Prototypes for functions in utils/mmgr/portalmem.c */
extern void EnablePortalManager(void);
extern bool PreCommit_Portals(bool isPrepare);
extern void AtAbort_Portals(void);
extern void AtCleanup_Portals(void);
extern void PortalErrorCleanup(void);
extern void AtSubCommit_Portals(SubTransactionId mySubid,
                                SubTransactionId parentSubid,
                                ResourceOwner parentXactOwner);
extern void AtSubAbort_Portals(SubTransactionId mySubid,
                               SubTransactionId parentSubid,
                               ResourceOwner myXactOwner,
                               ResourceOwner parentXactOwner);
extern void AtSubCleanup_Portals(SubTransactionId mySubid);
extern Portal CreatePortal(const char *name, bool allowDup, bool dupSilent);
extern Portal CreateNewPortal(void);
extern void PinPortal(Portal portal);
extern void UnpinPortal(Portal portal);
extern void MarkPortalActive(Portal portal);
extern void MarkPortalDone(Portal portal);
extern void MarkPortalFailed(Portal portal);
extern void PortalDrop(Portal portal, bool isTopCommit);
extern Portal GetPortalByName(const char *name);
extern void PortalDefineQuery(Portal portal,
                              const char *prepStmtName,
                              const char *sourceText,
                              CommandTag commandTag,
                              List *stmts,
                              CachedPlan *cplan);
extern PlannedStmt *PortalGetPrimaryStmt(Portal portal);
extern void PortalCreateHoldStore(Portal portal);
extern void PortalHashTableDeleteAll(void);
extern bool ThereAreNoReadyPortals(void);
extern void HoldPinnedPortals(void);
extern void ForgetPortalSnapshots(void);

#endif                          /* PORTAL_H */