Let ALTER TABLE Phase 2 routines manage the relation pointer

Struct AlteredRelationInfo gains a new Relation member, to be used only
by Phase 2 (ATRewriteCatalogs); this allows ATExecCmd() subroutines open
and close the relation internally.

A future commit will use this facility to implement an ALTER TABLE
subcommand that closes and reopens the relation across transaction
boundaries.

(It is possible to keep the relation open past phase 2 to be used by
phase 3 instead of having to reopen it that point, but there are some
minor complications with that; it's not clear that there is much to be
won from doing that, though.)

Author: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/20200803234854[email protected]

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 3349bcfaa747aa5ab6b18a79cf5d05655b51f8f3..bf7fd6e8ae665d94f8664a181599a819a1c8eadd 100644 (file)
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -157,6 +157,8 @@ typedef struct AlteredTableInfo
    Oid         relid;          /* Relation to work on */
    char        relkind;        /* Its relkind */
    TupleDesc   oldDesc;        /* Pre-modification tuple descriptor */
+   /* Transiently set during Phase 2, normally set to NULL */
+   Relation    rel;
    /* Information saved by Phase 1 for Phase 2: */
    List       *subcmds[AT_NUM_PASSES]; /* Lists of AlterTableCmd */
    /* Information saved by Phases 1/2 for Phase 3: */
@@ -354,7 +356,7 @@ static void ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
                      AlterTableUtilityContext *context);
 static void ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode,
                              AlterTableUtilityContext *context);
-static void ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
+static void ATExecCmd(List **wqueue, AlteredTableInfo *tab,
                      AlterTableCmd *cmd, LOCKMODE lockmode, int cur_pass,
                      AlterTableUtilityContext *context);
 static AlterTableCmd *ATParseTransformCmd(List **wqueue, AlteredTableInfo *tab,
@@ -4569,7 +4571,6 @@ ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode,
        {
            AlteredTableInfo *tab = (AlteredTableInfo *) lfirst(ltab);
            List       *subcmds = tab->subcmds[pass];
-           Relation    rel;
            ListCell   *lcmd;
 
            if (subcmds == NIL)
@@ -4578,10 +4579,10 @@ ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode,
            /*
             * Appropriate lock was obtained by phase 1, needn't get it again
             */
-           rel = relation_open(tab->relid, NoLock);
+           tab->rel = relation_open(tab->relid, NoLock);
 
            foreach(lcmd, subcmds)
-               ATExecCmd(wqueue, tab, rel,
+               ATExecCmd(wqueue, tab,
                          castNode(AlterTableCmd, lfirst(lcmd)),
                          lockmode, pass, context);
 
@@ -4593,7 +4594,11 @@ ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode,
            if (pass == AT_PASS_ALTER_TYPE)
                ATPostAlterTypeCleanup(wqueue, tab, lockmode);
 
-           relation_close(rel, NoLock);
+           if (tab->rel)
+           {
+               relation_close(tab->rel, NoLock);
+               tab->rel = NULL;
+           }
        }
    }
 
@@ -4619,11 +4624,12 @@ ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode,
  * ATExecCmd: dispatch a subcommand to appropriate execution routine
  */
 static void
-ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
+ATExecCmd(List **wqueue, AlteredTableInfo *tab,
          AlterTableCmd *cmd, LOCKMODE lockmode, int cur_pass,
          AlterTableUtilityContext *context)
 {
    ObjectAddress address = InvalidObjectAddress;
+   Relation    rel = tab->rel;
 
    switch (cmd->subtype)
    {
@@ -5730,6 +5736,7 @@ ATGetQueueEntry(List **wqueue, Relation rel)
     */
    tab = (AlteredTableInfo *) palloc0(sizeof(AlteredTableInfo));
    tab->relid = relid;
+   tab->rel = NULL;            /* set later */
    tab->relkind = rel->rd_rel->relkind;
    tab->oldDesc = CreateTupleDescCopyConstr(RelationGetDescr(rel));
    tab->newrelpersistence = RELPERSISTENCE_PERMANENT;