Minimize slot creation for multi-inserts of pg_shdepend

When doing multiple insertions in pg_shdepend for the copy of
dependencies from a template database in CREATE DATABASE, the same
number of slots would have been created and used all the time.  As the
number of items to insert is not known in advance, this makes most of
the slots created for nothing.  This improves the slot handling so as
slot creation only happens when needed, minimizing the overhead of the
operation.

Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200731024148.GB3317@paquier.xyz

diff --git a/src/backend/catalog/pg_shdepend.c b/src/backend/catalog/pg_shdepend.c
index ef2b87927ceb5ee99168643a8a531ec6993e5be1..30b234e90e124f4554e8a08a036f8a2cf7789ee5 100644 (file)
--- a/src/backend/catalog/pg_shdepend.c
+++ b/src/backend/catalog/pg_shdepend.c
@@ -809,15 +809,19 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
    int         slotCount;
    CatalogIndexState indstate;
    TupleTableSlot **slot;
-   int         nslots;
+   int         nslots,
+               max_slots;
+   bool        slot_init = true;
 
    sdepRel = table_open(SharedDependRelationId, RowExclusiveLock);
    sdepDesc = RelationGetDescr(sdepRel);
 
-   nslots = MAX_PGSHDEPEND_INSERT_BYTES / sizeof(FormData_pg_shdepend);
-   slot = palloc(sizeof(TupleTableSlot *) * nslots);
-   for (int i = 0; i < nslots; i++)
-       slot[i] = MakeSingleTupleTableSlot(sdepDesc, &TTSOpsHeapTuple);
+   /*
+    * Allocate the slots to use, but delay initialization until we know that
+    * they will be used.
+    */
+   max_slots = MAX_PGSHDEPEND_INSERT_BYTES / sizeof(FormData_pg_shdepend);
+   slot = palloc(sizeof(TupleTableSlot *) * max_slots);
 
    indstate = CatalogOpenIndexes(sdepRel);
 
@@ -842,6 +846,9 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
    {
        Form_pg_shdepend shdep;
 
+       if (slot_init)
+           slot[slotCount] = MakeSingleTupleTableSlot(sdepDesc, &TTSOpsHeapTuple);
+
        ExecClearTuple(slot[slotCount]);
 
        shdep = (Form_pg_shdepend) GETSTRUCT(tup);
@@ -858,10 +865,11 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
        slotCount++;
 
        /* If slots are full, insert a batch of tuples */
-       if (slotCount == nslots)
+       if (slotCount == max_slots)
        {
            CatalogTuplesMultiInsertWithInfo(sdepRel, slot, slotCount, indstate);
            slotCount = 0;
+           slot_init = false;
        }
    }
 
@@ -874,6 +882,8 @@ copyTemplateDependencies(Oid templateDbId, Oid newDbId)
    CatalogCloseIndexes(indstate);
    table_close(sdepRel, RowExclusiveLock);
 
+   /* Drop only the number of slots used */
+   nslots = slot_init ? slotCount : max_slots;
    for (int i = 0; i < nslots; i++)
        ExecDropSingleTupleTableSlot(slot[i]);
    pfree(slot);