[tracker/wip/carlosg/hotdoc: 5/40] docs: Port "implementation limits" document to markdown

[Carlos Garnacho <carlosg src gnome org>]

commit 65e60931734a98663fe85a90f91efbee3d74afeb
Author: Carlos Garnacho <carlosg gnome org>
Date:   Thu May 27 18:32:48 2021 +0200

    docs: Port "implementation limits" document to markdown

 docs/reference/libtracker-sparql/limits.md   |  84 +++++++++++++++++
 docs/reference/libtracker-sparql/limits.xml  | 132 ---------------------------
 docs/reference/libtracker-sparql/meson.build |   1 +
 3 files changed, 85 insertions(+), 132 deletions(-)
---
diff --git a/docs/reference/libtracker-sparql/limits.md b/docs/reference/libtracker-sparql/limits.md
new file mode 100644
index 000000000..d81d1febc
--- /dev/null
+++ b/docs/reference/libtracker-sparql/limits.md
@@ -0,0 +1,84 @@
+Title: Implementation limits
+Slug: implementation-limits
+
+Tracker is implemented on top of SQLite, and all of its benefits and
+[limits](https://www.sqlite.org/limits.html) apply. This
+document will break down how those limits apply to Tracker. Depending on
+your distributor, the limits might be changed via SQLite build-time
+options.
+
+## Maximum string length
+
+Tracker can store by default up to 1 GiB in a text field by default. This
+limit is controlled by `SQLITE_MAX_LENGTH`
+
+## Maximum number of properties
+
+The maximum amount of properties in the domain of a single class, and
+the maximum number of global fulltext-search properties in the ontology
+are limited by `SQLITE_MAX_COLUMN` (defaults to 2000).
+
+## Underlying parser limits
+
+SQLite defines some limits to its parser. Maximum query length is 1 MiB,
+its maximum expression tree depth is 1000, and it allows up to 500
+elements in `UNION / INTERSECT / EXCEPT` statements.
+
+There is no straightforward relation between the SPARQL query and the
+SQL one, so it is undefined how this limit applies to SPARQL queries.
+Query and ontology simplifications may help reduce its likelyhood.
+
+These SQLite limits are controlled by
+`SQLITE_MAX_SQL_LENGTH`, `SQLITE_MAX_EXPR_DEPTH` and
+`SQLITE_MAX_COMPOUND_SELECT`. The SPARQL parser
+is not affected in itself by these limits.
+
+## Maximum number of tables in a `JOIN`
+
+SQLite does not allow Join operations containing more than 64 tables.
+This limit may impact complex Tracker queries in various ways, most
+likely culprits are operations across the whole union graph, and
+access to multi-valued properties.
+
+Such queries can be tweaked (e.g. specifying a graph, or reducing
+the amount of accessed properties) or split into multiple ones to
+bypass this limit.
+
+## Maximum number or arguments in a function
+
+SQLite defines a limit of 100, controlled by `SQLITE_MAX_FUNCTION_ARG`.
+This affects all builtin and extra functions with variable arguments,
+e.g. `coalesce`.
+
+## Limits on the number of graphs
+
+SQLite has a limit on the number of databases that can be attached,
+defined by `SQLITE_MAX_LIMIT_ATTACHED` (defaults to 10, maximum 128).
+
+Tracker uses attached databases to implement its support for multiple
+graphs, so the maximum amount of graphs for a given [class@Tracker.SparqlConnection]
+is equally restricted.
+
+## Limits on glob search
+
+SQLite defines a maximum length of 50 KiB for GLOB patterns. This
+reflects in `CONTAINS / STRSTARTS / STRENDS`.
+SPARQL syntax.
+
+## Limits on the number of parameters in a statement
+
+SQLite defines a maximum of 999 parameters to be passed as arguments
+to a statement, controlled by `SQLITE_MAX_VARIABLE_NUMBER`.
+[class@Tracker.SparqlStatement] has the same limit.
+
+## Maximum number of pages in a database
+
+SQLite sets a default restriction of 1073741823 pages to
+database file size (about 8 TB with Tracker settings). This limit
+applies per graph.
+
+## Type formats and precision
+
+Integers are 64 bit wide. Floating point numbers have IEEE764
+double precision. Dates/times have microsecond precision, and may
+range between 0001-01-01 00:00:00 and 9999-12-31 23:59:59.
diff --git a/docs/reference/libtracker-sparql/meson.build b/docs/reference/libtracker-sparql/meson.build
index ba22fb2e5..8e9f5b149 100644
--- a/docs/reference/libtracker-sparql/meson.build
+++ b/docs/reference/libtracker-sparql/meson.build
@@ -1,5 +1,6 @@
 content = [
   'overview.md',
+  'limits.md',
   'tutorial.md',
 ]