[sqlite3] lazy creation of result rows

[erlend-aasland]

BPO	44976
Nosy	@berkerpeksag, @serhiy-storchaka, @pablogsal, @erlend-aasland
PRs	bpo-44976: Lazy creation of sqlite3 result rows #27884

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-08-25.10:29:18.621>
created_at = <Date 2021-08-22.09:06:30.968>
labels = ['extension-modules', 'type-feature']
title = '[sqlite3] lazy creation of result rows'
updated_at = <Date 2021-08-25.10:29:18.620>
user = 'https://github.com/erlend-aasland'

bugs.python.org fields:

activity = <Date 2021-08-25.10:29:18.620>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-08-25.10:29:18.621>
closer = 'pablogsal'
components = ['Extension Modules']
creation = <Date 2021-08-22.09:06:30.968>
creator = 'erlendaasland'
dependencies = []
files = []
hgrepos = []
issue_num = 44976
keywords = ['patch']
message_count = 3.0
messages = ['400062', '400065', '400252']
nosy_count = 4.0
nosy_names = ['berker.peksag', 'serhiy.storchaka', 'pablogsal', 'erlendaasland']
pr_nums = ['27884']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44976'
versions = []

[erlend-aasland]

Currently, we build the first result row in the _pysqlite_query_execute() loop if sqlite3_step() returned SQLITE_ROW. When the user asks for a row (for example, using sqlite3.Cursor.fetchone()), this pre-built row is returned, and the next row is prepared.

Suggesting to lazily build result rows instead.

Cons:

• no result tuples are built unless sqlite3.Cursor.fetch*() is called
• no need to keep the next result row (tuple) in pysqlite_Cursor; rows are built on demand
• pysqlite_cursor_iternext() is vastly simplified (50% less lines of code)
• the main loop in _pysqlite_query_execute() is further simplified

Cons:

• code churn

git diff main --shortstat: 2 files changed, 29 insertions(+), 58 deletions(-)

[erlend-aasland]

Historical note:
The current behaviour was introduced in pysqlite v2.0.2 (May 21 2005), in order to be able to finalise statements and thus close cursors as soon as possible. However, that effect was cancelled just after a couple of months, by the addition of the LRU cache and the ability to reuse prepared statements.

[pablogsal]

New changeset 3df0fc8 by Erlend Egeberg Aasland in branch 'main':
bpo-44976: Lazy creation of sqlite3 result rows (GH-27884)
3df0fc8