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

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The initial implementation in commit 959b38d77 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 959b38d77 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
This commit is contained in:
Tom Lane 2024-07-29 12:17:24 -04:00
parent b3f0e0503f
commit 0f12905215
1 changed files with 25 additions and 3 deletions
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28
src/bin/pg_dump/pg_backup_archiver.c
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@ -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;
}
}
/*