Count individual SQL commands in pg_restore's --transaction-size mode.

tglsfdc · tglsfdc · commit 0f1290521504 · 2024-07-29T12:17:24.000-04:00
The initial implementation in commit 959b38d counted one action per TOC entry (except for some special cases for multi-blob BLOBS entries). This assumes that TOC entries are all about equally complex, but it turns out that that assumption doesn't hold up very well in binary-upgrade mode. For example, even after the previous commit I was able to cause backend bloat with tables having many inherited constraints. There may be other cases too. (Since no serious problems have been reported with --single-transaction mode, we can conclude that the backend copes well with psql's regular restore scripts; but before 959b38d we never ran binary-upgrade restores with multi-command transactions.) To fix, count multi-command TOC entries as N actions, allowing the transaction size to be scaled down when we hit a complex TOC entry. Rather than add a SQL parser to pg_restore, approximate "multi command" by counting semicolons in the TOC entry's defn string. This will be fooled by semicolons appearing in string literals --- but the error is in the conservative direction, so it doesn't seem worth working harder. The biggest risk is with function/procedure TOC entries, but we can just explicitly skip those. (This is undoubtedly a hack, and maybe someday we'll be able to revert it after fixing the backend's bloat issues or rethinking what pg_dump emits in binary upgrade mode. But that surely isn't a project for v17.) Thanks to Alexander Korotkov for the let's-count-semicolons idea. Per report from Justin Pryzby. Back-patch to v17 where txn_size mode was introduced. Discussion: https://postgr.es/m/ZqEND4ZcTDBmcv31@pryzbyj2023
diff --git a/src/bin/pg_dump/pg_backup_archiver.c b/src/bin/pg_dump/pg_backup_archiver.c
@@ -3827,10 +3827,32 @@ _printTocEntry(ArchiveHandle *AH, TocEntry *te, bool isData)
 	{
 		IssueACLPerBlob(AH, te);
 	}
-	else
+	else if (te->defn && strlen(te->defn) > 0)
 	{
-		if (te->defn && strlen(te->defn) > 0)
-			ahprintf(AH, "%s\n\n", te->defn);
+		ahprintf(AH, "%s\n\n", te->defn);
+
+		/*
+		 * If the defn string contains multiple SQL commands, txn_size mode
+		 * should count it as N actions not one.  But rather than build a full
+		 * SQL parser, approximate this by counting semicolons.  One case
+		 * where that tends to be badly fooled is function definitions, so
+		 * ignore them.  (restore_toc_entry will count one action anyway.)
+		 */
+		if (ropt->txn_size > 0 &&
+			strcmp(te->desc, "FUNCTION") != 0 &&
+			strcmp(te->desc, "PROCEDURE") != 0)
+		{
+			const char *p = te->defn;
+			int			nsemis = 0;
+
+			while ((p = strchr(p, ';')) != NULL)
+			{
+				nsemis++;
+				p++;
+			}
+			if (nsemis > 1)
+				AH->txnCount += nsemis - 1;
+		}
 	}
 
 	/*
