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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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10
src/interfaces/libpq/libpq-int.h
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@@ -434,7 +434,10 @@ struct pg_conn
bool nonblocking; /* whether this connection is using nonblock
* sending semantics */
PGpipelineStatus pipelineStatus; /* status of pipeline mode */
bool partialResMode; /* true if single-row or chunked mode */
bool singleRowMode; /* return current query result row-by-row? */
int maxChunkSize; /* return query result in chunks not exceeding
* this number of rows */
char copy_is_binary; /* 1 = copy binary, 0 = copy text */
int copy_already_done; /* # bytes already returned in COPY OUT */
PGnotify *notifyHead; /* oldest unreported Notify msg */
@@ -535,12 +538,13 @@ struct pg_conn
* and error_result is true, then we need to return a PGRES_FATAL_ERROR
* result, but haven't yet constructed it; text for the error has been
* appended to conn->errorMessage. (Delaying construction simplifies
* dealing with out-of-memory cases.) If next_result isn't NULL, it is a
* PGresult that will replace "result" after we return that one.
* dealing with out-of-memory cases.) If saved_result isn't NULL, it is a
* PGresult that will replace "result" after we return that one; we use
* that in partial-result mode to remember the query's tuple metadata.
*/
PGresult *result; /* result being constructed */
bool error_result; /* do we need to make an ERROR result? */
PGresult *next_result; /* next result (used in single-row mode) */
PGresult *saved_result; /* original, empty result in partialResMode */
/* Assorted state for SASL, SSL, GSS, etc */
const pg_fe_sasl_mech *sasl;