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Support retrieval of results in chunks with libpq.

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This patch generalizes libpq's existing single-row mode to allow
individual partial-result PGresults to contain up to N rows, rather
than always one row.  This reduces malloc overhead compared to plain
single-row mode, and it is very useful for psql's FETCH_COUNT feature,
since otherwise we'd have to add code (and cycles) to either merge
single-row PGresults into a bigger one or teach psql's
results-printing logic to accept arrays of PGresults.

To avoid API breakage, PQsetSingleRowMode() remains the same, and we
add a new function PQsetChunkedRowsMode() to invoke the more general
case.  Also, PGresults obtained the old way continue to carry the
PGRES_SINGLE_TUPLE status code, while if PQsetChunkedRowsMode() is
used then their status code is PGRES_TUPLES_CHUNK.  The underlying
logic is the same either way, though.

Daniel Vérité, reviewed by Laurenz Albe and myself (and whacked
around a bit by me, so any remaining bugs are my fault)

Discussion: https://postgr.es/m/CAKZiRmxsVTkO928CM+-ADvsMyePmU3L9DQCa9NwqjvLPcEe5QA@mail.gmail.com
This commit is contained in:
Tom Lane
2024-04-06 20:41:32 -04:00
parent 92641d8d65
commit 4643a2b265
10 changed files with 249 additions and 92 deletions
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3
src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
View File

@ -1248,8 +1248,9 @@ libpqrcv_exec(WalReceiverConn *conn, const char *query,
switch (PQresultStatus(pgres))
{
case PGRES_SINGLE_TUPLE:
case PGRES_TUPLES_OK:
case PGRES_SINGLE_TUPLE:
case PGRES_TUPLES_CHUNK:
walres->status = WALRCV_OK_TUPLES;
libpqrcv_processTuples(pgres, walres, nRetTypes, retTypes);
break;