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Fast ALTER TABLE ADD COLUMN with a non-NULL default
Currently adding a column to a table with a non-NULL default results in a rewrite of the table. For large tables this can be both expensive and disruptive. This patch removes the need for the rewrite as long as the default value is not volatile. The default expression is evaluated at the time of the ALTER TABLE and the result stored in a new column (attmissingval) in pg_attribute, and a new column (atthasmissing) is set to true. Any existing row when fetched will be supplied with the attmissingval. New rows will have the supplied value or the default and so will never need the attmissingval. Any time the table is rewritten all the atthasmissing and attmissingval settings for the attributes are cleared, as they are no longer needed. The most visible code change from this is in heap_attisnull, which acquires a third TupleDesc argument, allowing it to detect a missing value if there is one. In many cases where it is known that there will not be any (e.g. catalog relations) NULL can be passed for this argument. Andrew Dunstan, heavily modified from an original patch from Serge Rielau. Reviewed by Tom Lane, Andres Freund, Tomas Vondra and David Rowley. Discussion: https://postgr.es/m/31e2e921-7002-4c27-59f5-51f08404c858@2ndQuadrant.com
This commit is contained in:
Andrew Dunstan
82
src/backend/utils/cache/relcache.c
vendored
82
src/backend/utils/cache/relcache.c
vendored
@ -36,6 +36,7 @@
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#include "access/nbtree.h"
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#include "access/reloptions.h"
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#include "access/sysattr.h"
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#include "access/tupdesc_details.h"
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#include "access/xact.h"
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#include "access/xlog.h"
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#include "catalog/catalog.h"
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@ -79,6 +80,7 @@
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#include "storage/smgr.h"
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#include "utils/array.h"
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#include "utils/builtins.h"
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#include "utils/datum.h"
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#include "utils/fmgroids.h"
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#include "utils/inval.h"
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#include "utils/lsyscache.h"
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@ -495,6 +497,7 @@ RelationBuildTupleDesc(Relation relation)
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int need;
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TupleConstr *constr;
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AttrDefault *attrdef = NULL;
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AttrMissing *attrmiss = NULL;
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int ndef = 0;
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/* copy some fields from pg_class row to rd_att */
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@ -540,15 +543,17 @@ RelationBuildTupleDesc(Relation relation)
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while (HeapTupleIsValid(pg_attribute_tuple = systable_getnext(pg_attribute_scan)))
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{
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Form_pg_attribute attp;
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int attnum;
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attp = (Form_pg_attribute) GETSTRUCT(pg_attribute_tuple);
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if (attp->attnum <= 0 ||
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attp->attnum > relation->rd_rel->relnatts)
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attnum = attp->attnum;
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if (attnum <= 0 || attnum > relation->rd_rel->relnatts)
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elog(ERROR, "invalid attribute number %d for %s",
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attp->attnum, RelationGetRelationName(relation));
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memcpy(TupleDescAttr(relation->rd_att, attp->attnum - 1),
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memcpy(TupleDescAttr(relation->rd_att, attnum - 1),
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attp,
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ATTRIBUTE_FIXED_PART_SIZE);
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@ -556,6 +561,7 @@ RelationBuildTupleDesc(Relation relation)
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if (attp->attnotnull)
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constr->has_not_null = true;
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/* If the column has a default, fill it into the attrdef array */
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if (attp->atthasdef)
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{
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if (attrdef == NULL)
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@ -563,10 +569,63 @@ RelationBuildTupleDesc(Relation relation)
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MemoryContextAllocZero(CacheMemoryContext,
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relation->rd_rel->relnatts *
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sizeof(AttrDefault));
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attrdef[ndef].adnum = attp->attnum;
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attrdef[ndef].adnum = attnum;
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attrdef[ndef].adbin = NULL;
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ndef++;
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}
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/* Likewise for a missing value */
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if (attp->atthasmissing)
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{
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Datum missingval;
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bool missingNull;
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/* Do we have a missing value? */
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missingval = heap_getattr(pg_attribute_tuple,
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Anum_pg_attribute_attmissingval,
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pg_attribute_desc->rd_att,
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&missingNull);
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if (!missingNull)
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{
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/* Yes, fetch from the array */
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MemoryContext oldcxt;
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bool is_null;
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int one = 1;
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Datum missval;
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if (attrmiss == NULL)
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attrmiss = (AttrMissing *)
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MemoryContextAllocZero(CacheMemoryContext,
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relation->rd_rel->relnatts *
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sizeof(AttrMissing));
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missval = array_get_element(missingval,
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1,
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&one,
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-1,
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attp->attlen,
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attp->attbyval,
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attp->attalign,
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&is_null);
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Assert(!is_null);
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if (attp->attbyval)
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{
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/* for copy by val just copy the datum direct */
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attrmiss[attnum - 1].ammissing = missval;
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}
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else
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{
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/* otherwise copy in the correct context */
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oldcxt = MemoryContextSwitchTo(CacheMemoryContext);
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attrmiss[attnum - 1].ammissing = datumCopy(missval,
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attp->attbyval,
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attp->attlen);
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MemoryContextSwitchTo(oldcxt);
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}
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attrmiss[attnum - 1].ammissingPresent = true;
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}
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}
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need--;
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if (need == 0)
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break;
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@ -607,7 +666,8 @@ RelationBuildTupleDesc(Relation relation)
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/*
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* Set up constraint/default info
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*/
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if (constr->has_not_null || ndef > 0 || relation->rd_rel->relchecks)
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if (constr->has_not_null || ndef > 0 ||
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attrmiss || relation->rd_rel->relchecks)
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{
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relation->rd_att->constr = constr;
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@ -624,6 +684,8 @@ RelationBuildTupleDesc(Relation relation)
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else
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constr->num_defval = 0;
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constr->missing = attrmiss;
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if (relation->rd_rel->relchecks > 0) /* CHECKs */
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{
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constr->num_check = relation->rd_rel->relchecks;
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@ -4063,10 +4125,6 @@ AttrDefaultFetch(Relation relation)
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systable_endscan(adscan);
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heap_close(adrel, AccessShareLock);
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if (found != ndef)
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elog(WARNING, "%d attrdef record(s) missing for rel %s",
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ndef - found, RelationGetRelationName(relation));
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}
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/*
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@ -4405,7 +4463,7 @@ RelationGetIndexList(Relation relation)
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*/
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if (!IndexIsValid(index) || !index->indisunique ||
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!index->indimmediate ||
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!heap_attisnull(htup, Anum_pg_index_indpred))
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!heap_attisnull(htup, Anum_pg_index_indpred, NULL))
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continue;
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/* Check to see if is a usable btree index on OID */
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@ -4700,7 +4758,7 @@ RelationGetIndexExpressions(Relation relation)
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/* Quick exit if there is nothing to do. */
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if (relation->rd_indextuple == NULL ||
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heap_attisnull(relation->rd_indextuple, Anum_pg_index_indexprs))
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heap_attisnull(relation->rd_indextuple, Anum_pg_index_indexprs, NULL))
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return NIL;
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/*
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@ -4762,7 +4820,7 @@ RelationGetIndexPredicate(Relation relation)
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/* Quick exit if there is nothing to do. */
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if (relation->rd_indextuple == NULL ||
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heap_attisnull(relation->rd_indextuple, Anum_pg_index_indpred))
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heap_attisnull(relation->rd_indextuple, Anum_pg_index_indpred, NULL))
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return NIL;
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/*
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